#include <bits/stdc++.h>
using namespace std;
template <class t>
inline t read(t &x) {
  char c = getchar();
  bool f = 0;
  x = 0;
  while (!isdigit(c)) f |= c == '-', c = getchar();
  while (isdigit(c)) x = (x << 1) + (x << 3) + (c ^ 48), c = getchar();
  if (f) x = -x;
  return x;
}
template <class t>
inline void write(t x) {
  if (x < 0)
    putchar('-'), write(-x);
  else {
    if (x > 9) write(x / 10);
    putchar('0' + x % 10);
  }
}
const int N = 2e5 + 5;
int n, v[N], col[N], fl;
vector<int> ans, p[2];
vector<pair<int, int>> a[N], g[N];
void dfs(int x, int c) {
  v[x] = 1;
  col[x] = c;
  p[c].push_back(x);
  for (auto o : g[x]) {
    int y = o.first, w = o.second;
    if (!v[y])
      dfs(y, c ^ w);
    else if (col[y] ^ c ^ w)
      fl = 0;
  }
}
void doit() {
  read(n);
  ans.clear();
  for (int i = 1; i <= n; i++) a[i].clear(), g[i].clear(), v[i] = 0;
  for (int o : {0, 1})
    for (int i = 1, x; i <= n; i++) a[read(x)].emplace_back(o, i);
  for (int i = 1; i <= n; i++) {
    if (a[i].size() < 2) return puts("-1"), void();
    int x = a[i][0].second, y = a[i][1].second,
        w = a[i][0].first == a[i][1].first;
    g[x].emplace_back(y, w), g[y].emplace_back(x, w);
  }
  fl = 1;
  for (int i = 1; i <= n; i++)
    if (!v[i]) {
      p[0].clear();
      p[1].clear();
      dfs(i, 0);
      if (p[0].size() > p[1].size()) swap(p[0], p[1]);
      for (int x : p[0]) ans.push_back(x);
    }
  if (!fl) return puts("-1"), void();
  write(ans.size());
  puts("");
  for (int x : ans) write(x), putchar(' ');
  puts("");
}
signed main() {
  int t;
  read(t);
  while (t--) doit();
}
