#include<bits/stdc++.h>
#define x first
#define y second
#define int long long
using namespace std;
typedef pair<int,int> PII;
typedef pair<PII,PII>PIII;
const int N=4e5+10,M=N*2,INF = 1e15,mod = 998244353;
int h[M],e[M],ne[M],idx;
vector<PIII>ans;
bool st1[M],st2[N];
int sz[N],n;
PII f[N];
void add(int a,int b)
{
    e[idx]=b,ne[idx]=h[a],h[a]=idx++;
}
void dfs(int u,int fa)
{
    st2[u]=true;
    PII last={0,0};
    for(int i = h[u];~i;i=ne[i])
    {
        if(st1[i])  continue;
        int j = e[i];
        sz[u]++;
        st1[i]=st1[i^1]=true;
        PII tmp = {u,j};
        if(tmp.x>2*n)   swap(tmp.x,tmp.y);        
        if(!st2[j])
        {
            dfs(j,u);
            sz[u]+=sz[j];

            if(sz[j]&1) ans.push_back({tmp,f[j]});
            else
            {
                if(last.x)
                {
                    ans.push_back({last,tmp});
                    last = {0,0};
                }
                else    last = tmp;
            }
        }
        else
        {
            if(last.x) 
            {
                ans.push_back({last,tmp});
                last = {0,0};
            }
            else    last = tmp;
        }
    }
    if(sz[u]&1) f[u]=last;
}
void solve()
{
    int id1=0,id2=0;
    for(int i=0;i<idx;i++)st1[i]=0;
    idx=0;
    cin>>n;
    ans.clear();
    for(int i=1;i<=4*n;i++)
    {
        h[i]=-1;
        sz[i]=st2[i]=0,f[i]={INF,INF};
    }
    map<int,int>mp1,mp2;
    map<PII,int>mp;
    for(int i=1;i<=2*n;i++)
    {
        int x,y;
        cin>>x>>y;
        if(!mp1.count(x))   mp1[x]=++id1;
        if(!mp2.count(y))   mp2[y]=++id2+2*n;
        add(mp1[x],mp2[y]),add(mp2[y],mp1[x]);
        mp[{mp1[x],mp2[y]}]=i;
    }
    PII last={0,0};
    int res=0;
    for(int i=1;i<=id1;i++)
        if(!sz[i])
        {
            dfs(i,0);
            res+=sz[i]>>1;
            if(sz[i]&1)
            {
                if(last.x)
                {
                    ans.push_back({last,f[i]});
                    last={0,0};
                }
                else    last=f[i];
            }
        }
    cout<<res<<"\n";
    for(auto it : ans)
        cout<<mp[it.x]<<" "<<mp[it.y]<<"\n";
    
}

signed main() {
	int T=1;
	cin.tie(0)->sync_with_stdio(false);
    cin>>T;
	while(T--)
	{
        solve();
	}
	return 0;
}