#include<bits/stdc++.h>
using namespace std;
#define V vector
#define pb push_back
#define fi first
#define se second
#define all1(x) (x).begin() + 1, (x).end()
typedef long long LL;
typedef pair<int, int> pi;
typedef pair<LL, LL> pl;


void solve() {
    int n; cin >> n;
    V<LL> a(n + 1);
    for(int i = 1; i <= n; i++) cin >> a[i];
    if(n > 200) {
        cout << -1 << endl;
        return;
    }

    priority_queue<pl> pq;

    for(int i = 1; i <= n; i++) {
        pq.push({a[i], i});
    }

    V<pl> ans;

    while(pq.size()) {
        auto [x, xid] = pq.top();
        pq.pop();
        auto [y, yid] = pq.top();
        pq.pop();
        if(__lg(x) != __lg(y) || pq.size() && __lg(pq.top().fi) == __lg(x)) {
            cout << -1 << endl;
            return;
        }

        ans.pb({xid, (x - (x ^ y))});
        if(x != y) pq.push({x ^ y, xid});
    }
    cout << ans.size() << endl;
    for(auto [x, y] : ans) cout << x << " " << y << endl;
}


int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    int t; cin >> t;
    while(t--) solve();
}