#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")

////////////////////////////////////////////////////////////////////////////////
template <unsigned M_> struct ModInt {
  static constexpr unsigned M = M_;
  unsigned x;
  constexpr ModInt() : x(0U) {}
  constexpr ModInt(unsigned x_) : x(x_ % M) {}
  constexpr ModInt(unsigned long long x_) : x(x_ % M) {}
  constexpr ModInt(int x_) : x(((x_ %= static_cast<int>(M)) < 0) ? (x_ + static_cast<int>(M)) : x_) {}
  constexpr ModInt(long long x_) : x(((x_ %= static_cast<long long>(M)) < 0) ? (x_ + static_cast<long long>(M)) : x_) {}
  ModInt &operator+=(const ModInt &a) { x = ((x += a.x) >= M) ? (x - M) : x; return *this; }
  ModInt &operator-=(const ModInt &a) { x = ((x -= a.x) >= M) ? (x + M) : x; return *this; }
  ModInt &operator*=(const ModInt &a) { x = (static_cast<unsigned long long>(x) * a.x) % M; return *this; }
  ModInt &operator/=(const ModInt &a) { return (*this *= a.inv()); }
  ModInt pow(long long e) const {
    if (e < 0) return inv().pow(-e);
    ModInt a = *this, b = 1U; for (; e; e >>= 1) { if (e & 1) b *= a; a *= a; } return b;
  }
  ModInt inv() const {
    unsigned a = M, b = x; int y = 0, z = 1;
    for (; b; ) { const unsigned q = a / b; const unsigned c = a - q * b; a = b; b = c; const int w = y - static_cast<int>(q) * z; y = z; z = w; }
    assert(a == 1U); return ModInt(y);
  }
  ModInt operator+() const { return *this; }
  ModInt operator-() const { ModInt a; a.x = x ? (M - x) : 0U; return a; }
  ModInt operator+(const ModInt &a) const { return (ModInt(*this) += a); }
  ModInt operator-(const ModInt &a) const { return (ModInt(*this) -= a); }
  ModInt operator*(const ModInt &a) const { return (ModInt(*this) *= a); }
  ModInt operator/(const ModInt &a) const { return (ModInt(*this) /= a); }
  template <class T> friend ModInt operator+(T a, const ModInt &b) { return (ModInt(a) += b); }
  template <class T> friend ModInt operator-(T a, const ModInt &b) { return (ModInt(a) -= b); }
  template <class T> friend ModInt operator*(T a, const ModInt &b) { return (ModInt(a) *= b); }
  template <class T> friend ModInt operator/(T a, const ModInt &b) { return (ModInt(a) /= b); }
  explicit operator bool() const { return x; }
  bool operator==(const ModInt &a) const { return (x == a.x); }
  bool operator!=(const ModInt &a) const { return (x != a.x); }
  friend std::ostream &operator<<(std::ostream &os, const ModInt &a) { return os << a.x; }
};
////////////////////////////////////////////////////////////////////////////////

constexpr unsigned MO = 998244353;
using Mint = ModInt<MO>;


// (without edge property)
// sub[u]: inside subtree at u, rooted at u
// bus[u]: outside subtree at u, rooted at par[u]
// tot[u]: rooted at u
// T: monoid representing information of a subtree.
//   T::init()  should assign the identity.
//   T::pull(const T &l, const T &r)  should assign the product.
//   T::up(int u)  should attach vertex u to the product of the subtrees.
template <class T> struct ForestDP {
  int n;
  vector<vector<int>> graph;
  vector<int> par;
  vector<T> sub, bus, tot;

  ForestDP() : n(0) {}
  explicit ForestDP(int n_) : n(n_), graph(n_) {}
  void ae(int u, int v) {
    assert(0 <= u); assert(u < n);
    assert(0 <= v); assert(v < n);
    graph[u].push_back(v);
    graph[v].push_back(u);
  }
  void run() {
    par.assign(n, -2);
    sub.resize(n);
    bus.resize(n);
    tot.resize(n);
    for (int u = 0; u < n; ++u) if (par[u] == -2) {
      dfs0(u, -1);
      dfs1(u, -1);
    }
  }
  void dfs0(int u, int p) {
    par[u] = p;
    const int deg = graph[u].size();
    int w = -1;
    for (int j = deg; --j >= 0; ) {
      const int v = graph[u][j];
      if (p != v) {
        dfs0(v, u);
        if (~w) {
          bus[v].pull(sub[v], bus[w]);
        } else {
          bus[v] = sub[v];
        }
        w = v;
      }
    }
    if (~w) {
      sub[u] = bus[w];
    } else {
      sub[u].init();
    }
    sub[u].up(u);
  }
  void dfs1(int u, int p) {
    const int deg = graph[u].size();
    int v = -1;
    for (int j = 0; j < deg; ++j) {
      const int w = graph[u][j];
      if (p != w) {
        if (~v) {
          bus[v].pull(tot[v], bus[w]);
          bus[v].up(u);
          tot[w].pull(tot[v], sub[v]);
          dfs1(v, u);
        } else {
          if (~p) {
            tot[w] = bus[u];
          } else {
            tot[w].init();
          }
        }
        v = w;
      }
    }
    if (~v) {
      bus[v] = tot[v];
      bus[v].up(u);
      tot[u].pull(tot[v], sub[v]);
      dfs1(v, u);
    } else {
      if (~p) {
        tot[u] = bus[u];
      } else {
        tot[u].init();
      }
    }
    tot[u].up(u);
  }
};


/*
  leaf-edge: w[i]: X[i]-Y[i]
  other: 1
  
  contributions
  1^2: const
  w[i]^2: (N-1)
  1 * 1: const
  w[i] * 1: depends on i
  w[i] * w[j]: 1
    2 \sum[i<j] w[i] w[j] = (\sum[i] w[i])^2 - \sum[i] w[i]^2
*/

int N, C;
vector<int> A, B;

vector<int> deg;

struct Data {
  Int s0, s1;
  Mint s2;
  
  Data() {}
  void init() {
    s0 = s1 = 0;
    s2 = 0;
  }
  void pull(const Data &a, const Data &b) {
    assert(this != &a);
    assert(this != &b);
    s0 = a.s0 + b.s0;
    s1 = a.s1 + b.s1;
    s2 = a.s2 + b.s2;
  }
  void up(int u) {
    s2 += s1;
    s1 += s0;
    s0 += 1;
  }
};

// merge (a[i], a[i] + d, a[i] + 2 d, ...)
template <class F> void mergeArithBrute(const vector<Int> &as, Int d, int n, F f) {
  const int asLen = as.size();
  priority_queue<Int, vector<Int>, greater<Int>> que;
  for (int i = 0; i < asLen; ++i) que.push(as[i]);
  for (int j = 0; j < n; ++j) {
    const Int b = que.top();
    que.pop();
    f(b);
    que.push(b + d);
  }
}

int main() {
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    scanf("%d%d", &N, &C);
    A.resize(N - 1);
    B.resize(N - 1);
    for (int i = 0; i < N - 1; ++i) {
      scanf("%d%d", &A[i], &B[i]);
      --A[i];
      --B[i];
    }
    
    Mint ans = 0;
    if (N == 2) {
      for (int c = N - 1; c <= C; ++c) {
        const Mint now = (Int)c * c;
        ans += now*now*now;
      }
    } else {
      deg.assign(N, 0);
      for (int i = 0; i < N - 1; ++i) {
        ++deg[A[i]];
        ++deg[B[i]];
      }
      int rt = -1;
      vector<int> X;
      for (int u = 0; u < N; ++u) {
        if (deg[u] == 1) {
          X.push_back(u);
        } else {
          if (!~rt) rt = u;
        }
      }
      const int L = X.size();
      
      ForestDP<Data> f(N), g(N);
      for (int i = 0; i < N - 1; ++i) {
        f.ae(A[i], B[i]);
        if (deg[A[i]] > 1 && deg[B[i]] > 1) {
          g.ae(A[i], B[i]);
        }
      }
      f.run();
      g.run();
// for(int u=0;u<N;++u)cerr<<u<<": "<<f.tot[u].s0<<" "<<f.tot[u].s1<<" "<<f.tot[u].s2<<"; "<<g.tot[u].s0<<" "<<g.tot[u].s1<<endl;
      
      // all 1
      Mint base = 0;
      for (int u = 0; u < N; ++u) {
        base += f.tot[u].s1;
        base += 2 * f.tot[u].s2;
      }
      base /= 2;
      
      // (N-2) w^2 + p w
      vector<Int> ps(L);
      for (int i = 0; i < L; ++i) {
        const int x = X[i];
        const int y = f.graph[x][0];
        ps[i] = 2 * g.tot[y].s1;
      }
// cerr<<"base = "<<base<<", ps = "<<ps<<endl;
      
      // diff: (N-2) (2w+1) + p
      for (int i = 0; i < L; ++i) {
        ps[i] += (N-2) * 3;
      }
      
      Mint now = base;
      int c = L;
      mergeArithBrute(ps, 2 * (N-2), C - (N-1) + 1, [&](Int t) -> void {
// cerr<<"c = "<<c<<": now = "<<now<<", t = "<<t<<endl;
        ans += now*now*now;
        now += t;
        // c -> c+1
        now += (c + c + 1);
        ++c;
      });
    }
    printf("%u\n", ans.x);
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}