#include<bits/stdc++.h>
#define Gc() getchar()
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define eb emplace_back
#define all(x) x.begin(),x.end()
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;
const int N=3e5+5,M=2000+5,K=1000+5,mod=998244353,Mod=mod-1;const db eps=1e-8;const int INF=1e9+7;mt19937 rnd(263082);
#define Tp template<typename T>
#define Ts template<typename T,typename... Ar>
namespace Debug{
	Tp void _debug(char* f,T t){cerr<<f<<'='<<t<<endl;}
	Ts void _debug(char* f,T x,Ar... y){while(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
	#ifdef LOCAL
	#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
	#else 
	#define gdb(...) void()
	#endif
}using namespace Debug;
int n,C;vector<int> S[N];
int siz[N];ll f1[N],f2[N];
void DP1(int x,int La){
	siz[x]=1;f1[x]=0;
	for(int i:S[x]) if(i^La) DP1(i,x),f1[x]+=f1[i]+siz[i],siz[x]+=siz[i];
}
void DP2(int x,int La){
	for(int i:S[x]) if(i^La) f2[i]=f2[x]+n-siz[i]+f1[x]-f1[i]-siz[i],DP2(i,x);
}
ll w[N];int wh;
void Solve(){
	int i,j;scanf("%d%d",&n,&C);
	for(i=1;i<=n;i++) S[i].clear();
	for(i=1;i<n;i++){
		int x,y;scanf("%d%d",&x,&y);
		S[x].emplace_back(y);S[y].emplace_back(x);
	}
	int rt=1;for(i=2;i<=n;i++) if(S[i].size()>=2) rt=i;
	DP1(rt,0);f2[rt]=0;DP2(rt,0);
	ll tot=0,ans=0;for(i=1;i<=n;i++) if(i^rt) tot+=(f1[i]%mod*(n-siz[i])+f2[i]%mod*siz[i])%mod;
	wh=0;for(i=1;i<=n;i++) if(i^rt&&S[i].size()==1) w[++wh]=f2[i];
	sort(w+1,w+wh+1);
	queue<ll> q;q.emplace(w[1]);int R=2;
	int add=0;
	for(i=n-1;i<=C;i++){
		ans+=1ll*tot*tot%mod*tot%mod;
		// gdb(tot);
		ll x=q.front();q.pop();
		tot+=2*(x+add)+n-1;tot%=mod;
		add++;
		while(R<=wh&&w[R]<x+n-2) q.emplace(w[R]),R++;
		q.emplace(x+n-2);
	}
	printf("%lld\n",ans%mod);
}
int main(){
	int t=1;
	scanf("%d",&t);
	while(t--) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}