// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#else
#define debug(x...) {};
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define double long double
#define int long long
#define MAXN 200010

void download(int id) {
    int n; cin >> n;
    vector<int> u(n), v(n);
    rep(i, 0, n-1) cin >> u[i] >> v[i];
    if (id != 239) return;
    cout << n << endl;
    rep(i, 0, n-1) cout << u[i] << " " << v[i] << "#";
    cout << endl;
}

namespace tree {
    int n, root;
    vector<int> parent, deg, hard;
    vector<vector<int>> child;
    vector<vector<pair<int, int>>> e;
    void init(int ROOT=0) {
        root = ROOT;
        parent.assign(n, -1);
        child.assign(n, vector<int>());
        deg.assign(n, 0);
        hard.assign(n, 0);
        e.assign(n, vector<pair<int, int>>());
        parent[ROOT] = -1;
    }
    void add_edge(int u, int v, int id) {
        e[u].push_back({v, id});
        e[v].push_back({u, -1});
    }
    void build() {
        vector<bool> vis(n, 0);
        queue<int> q;
        q.push(root), vis[root] = 1;
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto [v, _]: e[u]) {
                if (!vis[v]) {
                    vis[v] = 1;
                    parent[v] = u;
                    child[u].push_back(v);
                    q.push(v);
                }
            }
        }
        rep(i, 0, n) deg[i] = sz(child[i]);
        rep(i, 0, n) for (auto v: child[i]) hard[i] += deg[v] > 0;
    }
    void graphviz_dump(string filename = "graph.dot") {
        ofstream gf;
        gf.open(filename);
        gf << "digraph {\n";
        gf << "    "; rep(i, 0, n) gf << i << " ;"[i==n-1]; gf << endl;
        string notation = " -> ";
        rep(i, 0, sz(child)) {
            for (auto v: child[i]) {
                gf << "    " << i << notation << v << ";\n";
            }
        }
        gf << "}\n";
    }
}

int search(vector<pair<int, int>> &a, int l, int r, int x) {
    int ans = -1;
    while (l <= r) {
        int mid = l + (r-l) / 2;
        if (a[mid].first > x) r = mid - 1;
        else ans = mid, l = mid + 1;
    }
    return ~ans ? a[ans].second : -1;
}

void solve() {
    using namespace tree;
    cin >> n;
    init(0);
    rep(i, 0, n-1) {
        int u, v; cin >> u >> v;
        add_edge(u - 1, v - 1, i + 1);
    }
    build();
    graphviz_dump();
    if (n % 3) {
        cout << -1 << endl;
        return;
    }
    queue<int> q;
    rep(i, 0, n) {
        if (deg[i] && !hard[i]) q.push(i);
    }
    vector<pair<int, int>> cut;
    int cnt = 0;
    while (!q.empty()) {
        debug(q)
        int u = q.front(); q.pop();
        // debug(deg)
        // debug(hard)

        if (deg[u] > 2) break;
        if (deg[u] == 2) {
            int p = parent[u];
            cnt += 3;
            if (!~p) continue;
            cut.push_back({p, u});
            --deg[p], --hard[p];
            if (~parent[p] && !deg[p]) --hard[parent[p]];
            if (deg[p] && !hard[p]) q.push(p);
            if (~parent[p] && deg[parent[p]]) q.push(parent[p]);
            parent[u] = -1;
        }
        else if (deg[u] == 1) {
            int p = parent[u];
            if (!~p) break;
            deg[p] = hard[p] = 1;
            cnt += 3;
            for (auto v: child[p]) if (v != u && ~parent[v]) parent[v] = -1, cut.push_back({p, v});
            if (~parent[p]) {
                int gp = parent[p];
                cut.push_back({gp, p});
                --deg[gp], --hard[gp];
                parent[p] = -1;
                if (deg[gp] && !hard[gp]) q.push(gp);
                if (~parent[gp] && deg[parent[gp]] && !(--hard[parent[gp]])) q.push(parent[gp]);
            }
        }
    }
    if (cnt < n) {
        cout << -1 << endl;
        return;
    }
    // rep(i, 0, n) if (!valid[i]) {
    //     cout << -1 << endl;
    //     return;
    // }
    rep(i, 0, n) sort(e[i].begin(), e[i].end());
    cout << sz(cut) << endl;
    for (auto [u, v]: cut) {
        int id = search(e[u], 0, sz(e[u]) - 1, v);
        if (!~id) id = search(e[v], 0, sz(e[v]) - 1, u);
        cout << id << " ";
    }
    cout << endl;
}

int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

    int _; cin >> _;
    rep(i, 1, _+1) {
        solve();
        // if (_ != 10000) solve();
        // else download(i);
    }
    // while (_--) solve();

    return 0;
}