#include<bits/stdc++.h>
using namespace std;

#define rep(i, a, n) for(int i=(a); i<(n); ++i)
#define per(i, a, n) for(int i=(a); i>(n); --i)
#define pb emplace_back
#define mp make_pair
#define clr(a, b) memset(a, b, sizeof(a))
#define all(x) (x).begin(),(x).end()
#define lowbit(x) (x & -x)
#define fi first
#define se second
#define lson o<<1
#define rson o<<1|1
#define gmid l[o]+r[o]>>1

using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll, ll>;
using vi = vector<int>;

constexpr int mod = 998244353;
constexpr int inf = 0x3f3f3f3f;
constexpr int N = 2e5 + 10;

int T, n, m;
unordered_map<int, int> idx, idy;
bool vis[N<<1], used[N], dp[N<<1];
vector<pii> V[N<<1], ans;
int dep[N<<1];

int getid(unordered_map<int,int> &id, int v){
  if(!id.count(v)){
    id[v] = ++m;
    vis[m] = 0;
  }
  return id[v];
}

void dfs(int x, int from){
  vis[x] = 1;
  int pre = 0;
  for(auto &p : V[x]){
    int y = p.fi;
    int id = p.se;
    if(vis[y] && dep[y] < dep[x])  continue;
    if(!vis[y]){
      dep[y] = dep[x] + 1;
      dfs(y, id);
      if(dp[y]){
        if(pre){
          ans.pb(pre, id);
          used[pre] = used[id] = 1;
          pre = 0;
        } else {
          pre = id;
        }
      }
    } else {
      if(pre){
        ans.pb(pre, id);
        used[pre] = used[id] = 1;
        pre = 0;
      } else {
        pre = id;
      }
    }
  }


  if(pre){
    if(from){
      ans.pb(pre, from);
      used[pre] = used[from] = 1;
    }
    dp[x] = 0;
  } else {
    dp[x] = 1;
  }
}

void _main(){
  int x, y;
  idx.max_load_factor(0.25);
  idy.max_load_factor(0.25);

  cin >> T;
  while(T--){
    cin >> n;

    m = 0;
    rep(i, 1, n * 2 + 1){
      cin >> x >> y;
      x = getid(idx, x);
      y = getid(idy, y);
      V[x].pb(y, i);
      V[y].pb(x, i);

      used[i] = 0;
    }

    ans.clear();

    rep(i, 1, m + 1){
      if(!vis[i]){
        dep[i] = 0;
        dfs(i, 0);
      }
    }

    cout << ans.size() << '\n';

    int pre = 0;
    rep(i, 1, n * 2 + 1){
      if(!used[i]){
        if(pre){
          ans.pb(pre, i);
          pre = 0;
        } else {
          pre = i;
        }
      }
    }

    for(auto &p : ans){
      cout << p.fi << ' ' << p.se << '\n';
    }

    rep(i, 1, m + 1){
      V[i].clear();
    }
    idx.clear();
    idy.clear();
  }
}

int main(){
  ios::sync_with_stdio(0);
  cin.tie(0); cout.tie(0);
  _main();
  return 0;
}