#pragma warning (disable : 4996)
#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#include <string>
#include <set>
#include <map>
#include <cassert>
#include <ctime>
#include <iomanip>

using namespace std;
#define sz size()
#define all(x) x.begin(),x.end()
#define forn(i,n) for (int i = 0; i<int(n); i++)
#define mp make_pair
typedef long long ll;
typedef long double ld;
const ll INF = 1e9 + 9;

bool check(string &a, string &b, vector<int> &v, vector<int> &v1){
    if (a.sz != b.sz)
        return false;
    for (int i = 0; i < a.size(); i++){
        int x = a[i] - 'a', y = b[i] - 'a';
        if (v[x] != -1 && v[x] != y || v1[y]!=-1 && v1[y]!=x)
            return false;
        if (v[x] == -1){
            v[x] = y;
        }
        if (v1[y] == -1){
            v1[y] = x;
        }
    }
    return true;
}

void solve(){
    int n;
    cin >> n;
    vector<string> vs(n);
    for (int i = 0; i < vs.size(); i++){
        cin >> vs[i];
    }
    string str;
    cin >> str;
    vector<int> v(30, -1);
    vector<int> v1(30, -1);
    bool good = false;
    for (int i = 0; i < vs.size(); i++){
        vector<int> qv(30,-1), qv1(30,-1);
        if (check(vs[i], str, qv, qv1)){
            good = true;
            for (int j = 0; j < 30; j++){
                if (-2 == v[j] || -1 == qv[j])
                    continue;
                if (-1 == v[j]){
                    if (-1 == v1[qv[j]]){
                        v1[qv[j]] = j;
                        v[j] = qv[j];
                    }
                    else if (-2 == v1[qv[j]]){
                        v[j] = -2;
                    }
                    else if (v1[qv[j]] != j){
                        v[v1[qv[j]]] = -2;
                        v1[qv[j]] = -2;
                        v[j] = -2;
                    }
                }
                else if (v[j] != qv[j]){
                    v[j] = -2;
                }
            }
        }
    }
    string code;
    cin >> code;
    if (!good){
        cout << "IMPOSSIBLE" << endl;
        return;
    }
    for (int i = 0; i < code.size(); i++){
        int x = code[i] - 'a';
        if (v[x] == -2 || v[x] == -1)
            cout << "?";
        else
            cout << (char)(v[x] + 'a');
    }
    cout << endl;
}

int main(){
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);

    int test;
    cin >> test;
    for (int i = 0; i < test; i++){
        solve();
    }

    return 0;
}