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

typedef pair<int,int> pii;
#define fir first
#define sec second
const int maxn=1e5+10;

inline int read(){
    int x=0,w=1;char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')w=-1;c=getchar();}
    while(c<='9'&&c>='0'){x=(x<<3)+(x<<1)+c-'0';c=getchar();}
    return x*w;
}

int t;
int n;
pii p[maxn<<1];
vector<int> Vx,Vy;
struct edge{
    int to,nxt,id;
}e[maxn<<3];
int head[maxn<<2],cnt;
inline void add_edge(int u,int v,int id){
    e[++cnt].to=v,e[cnt].nxt=head[u],e[cnt].id=id,head[u]=cnt;
    e[++cnt].to=u,e[cnt].nxt=head[v],e[cnt].id=id,head[v]=cnt;
}
int dfn[maxn<<2],dfncnt;
bool f[maxn<<2];
vector<int> now[maxn<<2];
vector<pii> ans;
bool mark[maxn<<1];
void dfs(int u,int fa){
    dfn[u]=++dfncnt;
    int id_fa=-1;
    for(int i=head[u];i;i=e[i].nxt){
        int v=e[i].to;
        if(v==fa){
            id_fa=e[i].id;
            continue;
        }
        if(!dfn[v]){
            dfs(v,u);
            if(!f[v]) now[u].push_back(e[i].id);
        }
        else{
            if(dfn[v]>dfn[u]) now[u].push_back(e[i].id);
        }
    }
    for(int i=1;i<now[u].size();i+=2){
        ans.push_back(make_pair(now[u][i-1],now[u][i]));
    }
    if(id_fa!=-1&&((int)now[u].size()&1)){
        f[u]=1;
        ans.push_back(make_pair(now[u][now[u].size()-1],id_fa));
    }
}   
int main(){
    // freopen("geometry.in","r",stdin);
    // freopen("geometry.out","w",stdout);
    t=read();
    while(t--){
        n=read()*2;
        Vx.clear(),Vy.clear();
        for(int i=1;i<=n;++i){
            p[i].fir=read(),p[i].sec=read();
            Vx.push_back(p[i].fir),Vy.push_back(p[i].sec);
            mark[i]=0;
        }
        sort(Vx.begin(),Vx.end());
        Vx.erase(unique(Vx.begin(),Vx.end()),Vx.end());
        sort(Vy.begin(),Vy.end());
        Vy.erase(unique(Vy.begin(),Vy.end()),Vy.end());
        cnt=1;
        dfncnt=0;
        for(int i=1;i<=Vx.size()+Vy.size();++i){
            head[i]=0;
            dfn[i]=0,f[i]=0;
            now[i].clear();
        }
        for(int i=1;i<=n;++i){
            p[i].fir=lower_bound(Vx.begin(),Vx.end(),p[i].fir)-Vx.begin()+1;
            p[i].sec=lower_bound(Vy.begin(),Vy.end(),p[i].sec)-Vy.begin()+1+Vx.size();
            add_edge(p[i].fir,p[i].sec,i);
        }
        ans.clear();
        for(int i=1;i<=Vx.size()+Vy.size();++i){
            if(!dfn[i]) dfs(i,0);
        }
        printf("%ld\n",ans.size());
        for(pii i:ans){
            printf("%d %d\n",i.fir,i.sec);
            mark[i.fir]=1,mark[i.sec]=1;
        }
        for(int i=1,j=-1;i<=n;++i){
            if(mark[i]) continue;
            if(j==-1) j=i;
            else{
                printf("%d %d\n",i,j);
                j=-1;
            }
        }
    }
    return 0;
}