#include <bits/stdc++.h>
using namespace std;
int Hash[(int)(2e5 + 10) << 1], pos1[(int)(2e5 + 10) << 1],
    pos2[(int)(2e5 + 10) << 1];
int res[(int)(2e5 + 10) << 1];
int p[(int)(2e5 + 10) << 1];
bool mark[(int)(2e5 + 10) << 1];
int n;
void init() {
  memset(Hash, 0, (n * 2 + 5) * sizeof(int));
  memset(pos1, 0, (n * 2 + 5) * sizeof(int));
  memset(pos2, 0, (n * 2 + 5) * sizeof(int));
  memset(mark, false, (n * 2 + 5) * sizeof(int));
  memset(res, 0, (n * 2 + 5) * sizeof(int));
  for (int i = 1; i <= n * 2 + 5; i++) p[i] = i;
}
int find(int x) { return p[x] == x ? x : p[x] = find(p[x]); }
void unite(int x, int y) {
  x = find(x), y = find(y);
  if (x == y) return;
  p[x] = y;
}
int main() {
  ios::sync_with_stdio(0);
  int T;
  cin >> T;
  while (T--) {
    cin >> n;
    init();
    for (int i = 1; i <= n; i++) {
      int a;
      cin >> a;
      Hash[a]++;
      if (!pos1[a])
        pos1[a] = 1, pos2[a] = i;
      else
        unite(pos2[a], i + n), unite(pos2[a] + n, i);
    }
    for (int i = 1; i <= n; i++) {
      int tmp;
      cin >> tmp;
      Hash[tmp]++;
      if (!pos1[tmp])
        pos1[tmp] = 2, pos2[tmp] = i;
      else {
        if (pos1[tmp] == 2)
          unite(pos2[tmp], i + n), unite(pos2[tmp] + n, i);
        else if (pos1[tmp] == 1) {
          if (pos2[tmp] != i)
            unite(pos2[tmp], i), unite(pos2[tmp] + n, i + n);
          else
            p[i] = -1, p[i + n] = -1;
        }
        pos1[tmp] = -1;
      }
    }
    bool flag = true;
    for (int i = 1; i <= n; i++)
      if (Hash[i] != 2) {
        flag = false;
        break;
      }
    if (!flag) {
      cout << "-1" << endl;
      continue;
    }
    for (int i = 1; i <= 2 * n; i++)
      if (p[i] != -1) p[i] = find(p[i]);
    for (int i = 1; i <= n; i++)
      if (p[i] != -1) res[p[i]]++;
    int ans = 0;
    for (int i = 1; i <= n; i++) {
      if (p[i] == -1) continue;
      if (res[p[i]] > 0 && res[p[i + n]] >= 0) {
        if (res[p[i]] >= res[p[i + n]])
          res[p[i]] = -1;
        else
          res[p[i + n]] = -1;
      }
      if (res[p[i]] > 0) mark[i] = 1, ans++;
    }
    cout << ans << endl;
    for (int i = 1; i <= n; i++)
      if (mark[i]) cout << i << " ";
    cout << endl;
  }
  return 0;
}
