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

typedef long long ll; typedef unsigned long long ull;
const int INF = 1e9; const ll LLNF = 4e18;

template< class Tp > void chkmax( Tp &x , Tp y ) { x = max( x , y ); }
template< class Tp > void chkmin( Tp &x , Tp y ) { x = min( x , y ); }

void solve( ) {
	int m; cin >> m; m *= 2;
	vector< int > x( m + 1 ) , y( m + 1 ); for( int i = 1; i <= m; i ++ ) cin >> x[i] >> y[i];

	map< int , int > X , Y;
	auto id = [&] ( map< int , int > &Z , int z ) -> int { if( !Z.count( z ) ) Z[z] = ( int ) Z.size( ) + 1; return Z[z]; } ;
	for( int i = 1; i <= m; i ++ ) x[i] = id( X , x[i] ) , y[i] = id( Y , y[i] );

	int n = ( int ) X.size( ) + ( int ) Y.size( ); vector< vector< pair< int , int > > > G( n + 1 );
	for( int i = 1; i <= m; i ++ ) { int u = x[i] , v = ( int ) X.size( ) + y[i]; G[u].emplace_back( v , i ) , G[v].emplace_back( u , i ); }

	vector< bool > vis( n + 1 ); vector< int > used( m + 1 ); vector< vector< pair< int , int > > > H( n + m + 1 ); int pH = n;
	auto dfs = [&] ( auto dfs , int u ) -> void {
		vis[u] = true;
		for( auto [ v , i ] : G[u] ) {
			if( !vis[v] ) used[i] = true , dfs( dfs , v ) , H[u].emplace_back( v , i ) , H[v].emplace_back( u , i );
			else if( !used[i] ) used[i] = true , pH ++ , H[u].emplace_back( pH , i ) , H[pH].emplace_back( u , i );
		}
	} ;
	for( int u = 1; u <= n; u ++ ) if( !vis[u] ) dfs( dfs , u );

	vector< pair< int , int > > ans; vector< int > par( pH + 1 );
	auto efs = [&] ( auto efs , int u ) -> int {
		vector< int > res;
		for( auto [ v , i ] : H[u] ) if( v != par[u] ) {
			par[v] = u;
			int j = efs( efs , v );
			if( j ) ans.emplace_back( i , j ); else res.emplace_back( i );
		}
		while( ( int ) res.size( ) >= 2 ) {
			int i = res.back( ); res.pop_back( );
			int j = res.back( ); res.pop_back( );
			ans.emplace_back( i , j );
		}
		return ( res.empty( ) ) ? ( 0 ) : ( res[0] );
	} ;
	for( int u = 1; u <= pH; u ++ ) if( !par[u] ) efs( efs , u );

	set< int > trash; for( int i = 1; i <= m; i ++ ) trash.emplace( i ); for( auto [ i , j ] : ans ) trash.erase( i ) , trash.erase( j );

	cout << ( int ) ans.size( ) << "\n";
	for( auto [ i , j ] : ans ) cout << i << " " << j << "\n";
	while( ( int ) trash.size( ) >= 2 ) {
		int i = *trash.begin( ); trash.erase( i );
		int j = *trash.begin( ); trash.erase( j );
		cout << i << " " << j << "\n";
	}
}

int main( ) {
	ios::sync_with_stdio( 0 ), cin.tie( 0 ), cout.tie( 0 );
	int T; cin >> T; while( T -- ) solve( ); return 0;
}