import sys
for i in range(int(input())):
n,m = map(int,input().split())
ans = [0]*m
for j in range(n + n - 1):
s = input()
for k in range(len(s)):
ans[k] ^= ord(s[k])
ans = [chr(i) for i in ans]
x = ''.join(ans)
print(x)
sys.stdout.flush()#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)
#define vi vector<int>
#define vvi vector<vi>
#define int long long
void solve(){
int n,m;
cin>>n>>m;
n=(n<<1)-1;
char a[m+5];
char s[m+5];
rep(i,0,m){
a[i]=0;
}
rep(i,0,n){
cin>>s;
rep(j,0,m){
a[j]^=s[j];
}
}
rep(i,0,m){
cout<<a[i];
}
cout<<endl;
}
bool isprime(int n) {
bool is_prime = true;
if (n == 0 || n == 1) {
is_prime = false;
}
for (int i = 2; i <= n / 2; ++i) {
if (n % i == 0) {
is_prime = false;
break;
}
}
return is_prime;
}
int32_t main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int t;
cin>>t;
while(t--){
solve();
}
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