解法：

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
#define endl '\n'
bool cmp(string& a, string& b) {
	if (a.size() != b.size()) {
		return a.size() > b.size();
	}
	else {
		for (int i = 0; i < a.size(); i++) {
			if (a[i] != b[i]) {
				return a[i] > b[i];
			}
		}
	}
}
void solve() {
	int n, m;
	cin >> n >> m;
	string mxa, mxb, s;
	cin >> mxa;
	for (int i = 1; i < n; i++) {
		cin >> s;
		if (cmp(s, mxa)) {
			mxa = s;
		}
	}
	cin >> mxb;
	for (int i = 1; i < m; i++) {
		cin >> s;
		if (cmp(s, mxb)) {
			mxb = s;
		}
	}
	cout << mxa + mxb << endl;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	int t;
	cin >> t;
	while (t--) {
		solve();
	}
	return 0;
}