#include <bits/stdc++.h>
#define ll long long int
#define ld long double
using namespace std;

void mainSolve()
{
  string s;
  int n;
  cin >> n >> s;
  for (int i = 0; i < n; i += 2)
  {
    if (s[i] == '0' && s[i + 1] == '0')
      cout << 'A';
    else if (s[i] == '0' && s[i + 1] == '1')
      cout << 'T';
    else if (s[i] == '1' && s[i + 1] == '0')
      cout << 'C';
    else
      cout << 'G';
  }
  cout << endl;
}

int main()
{
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t;
  cin >> t;
  while (t--)
  {
    mainSolve();
  }
  return 0;
}