#include <bits/stdc++.h>
using namespace std;
int n, ct[400020], a[400020], b[400020], fa[400020], vis[400020];
vector<pair<int, int> > loc[400020];
unordered_map<int, vector<int> > mp;
vector<int> ans;
int find(int x) { return fa[x] == x ? x : fa[x] = find(fa[x]); }
inline void solve() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) loc[i].clear(), ct[i] = 0;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]), ++ct[a[i]];
    loc[a[i]].push_back({1, i});
  }
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &b[i]), ++ct[b[i]];
    loc[b[i]].push_back({2, i});
  }
  for (int i = 1; i <= n; ++i)
    if (ct[i] != 2) {
      puts("-1");
      return;
    }
  for (int i = 1; i <= n + n; ++i) fa[i] = i;
  for (int i = 1; i <= n; ++i) {
    auto a = loc[i][0], b = loc[i][1];
    if (a.first != b.first) {
      int p = find(a.second), q = find(b.second);
      if (p != q) fa[q] = p;
      p = find(a.second + n), q = find(b.second + n);
      if (p != q) fa[q] = p;
    } else {
      int p = find(a.second), q = find(b.second + n);
      if (p != q) fa[q] = p;
      p = find(a.second + n), q = find(b.second);
      if (p != q) fa[q] = p;
    }
  }
  mp.clear();
  for (int i = 1; i <= n; ++i)
    if (find(i) == find(i + n)) {
      puts("-1");
      return;
    } else {
      mp[find(i)].push_back(i);
      mp[find(i + n)].push_back(i + n);
    }
  ans.clear();
  for (int i = 1; i <= n; ++i) vis[i] = 0;
  for (int i = 1; i <= n; ++i)
    if (!vis[i]) {
      int cl = 0, cr = 0;
      for (auto v : mp[find(i)])
        if (v > n) ++cl;
      for (auto v : mp[find(i + n)])
        if (v > n) ++cr;
      if (cl < cr) {
        for (auto v : mp[find(i)]) {
          if (v > n) ans.push_back(v - n);
          vis[v > n ? v - n : v] = 1;
        }
      } else {
        for (auto v : mp[find(i + n)]) {
          if (v > n) ans.push_back(v - n);
          vis[v > n ? v - n : v] = 1;
        }
      }
    }
  printf("%d\n", ans.size());
  for (auto v : ans) printf("%d ", v);
  puts("");
}
int main() {
  int T = 1;
  scanf("%d", &T);
  while (T--) solve();
}
