#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, m, k;
  cin >> n >> m >> k;
  vector<string> s(n, string(m, ' '));
  if (n % 2 == 1) {
    k -= m / 2;
    n--;
    char c = 'n';
    for (int i = 0; i < m; i += 2) {
      s[n][i] = s[n][i + 1] = c;
      c = c == 'n' ? 'm' : 'n';
    }
  }
  if (k < 0 || k % 2 == 1) {
    cout << "NO\n";
    return;
  }
  int id = 0;
  array<char, 3> p = {'a', 'b', 'c'}, q = {'e', 'f', 'g'};
  for (int i = 0; i + 1 < n && 0 < k; i += 2) {
    for (int j = 0; j + 1 < m && 0 < k; j += 2) {
      s[i][j] = s[i][j + 1] = p[id];
      s[i + 1][j] = s[i + 1][j + 1] = p[(id + 1) % 3];
      swap(p, q);
      k -= 2;
    }
    id = (id + 2) % 3;
  }
  p = {'x', 'y', 'z'}, q = {'u', 'v', 'w'};
  for (int j = 0; j < m; j++) {
    for (int i = 0; i + 1 < n; i++) {
      if (s[i][j] == ' ' && s[i + 1][j] == ' ') {
        s[i][j] = s[i + 1][j] = p[id];
        swap(p, q);
        i++;
      }
    }
    id = (id + 1) % 3;
  }
  if (k != 0) {
    cout << "NO\n";
    return;
  }
  cout << "YES\n";
  for (auto i : s) cout << i << '\n';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
