func_code_string: /*   Creater :        ANSHUMAN MISHRA    All Losses In this World Are Due To Lack Of Ability.   If You Want To Curse Someone , Curse Your Own Weakness    */         #include <bits/stdc++.h>   using namespace std;      #define ll long long   #define pii pair<int,int>   #define yes cout<< YES n    #define no cout<< NO n       const int N=1e5+5;   int cnt[26][N];      signed main(){       ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);       #ifndef ONLINE_JUDGE           freopen( in.txt , r ,stdin);       #endif          int t=1;cin>>t;       while(t--){           int n,m;           cin>>n>>m;           for (int i=0;i<m;++i) for (int j=0;j<26;++j) cnt[j][i]=0;              string s;           for (int i=0;i<n;++i){               cin>>s;               for (int j=0;j<m;++j)                   cnt[s[j]- a ][j]++;           }           /*           for (int i=0;i<m;++i){               for (int j=0;j<26;++j){                   /*                   if (cnt[j][i]==1){                       ans+= (j+ a );                       break;                   }                   cout<<cnt[j][i]<<   ;               }               cout<<  n ;           }*/              for (int i=0;i<n-1;++i){               cin>>s;               for (int j=0;j<m;++j)                   cnt[s[j]- a ][j]--;           }                            string ans=  ;           for (int i=0;i<m;++i){               for (int j=0;j<26;++j){                                      if (cnt[j][i]==1){                       ans+= (j+ a );                       break;                   }                   //cout<<cnt[j][i]<<   ;               }               //cout<<  n ;           }           cout<<ans<<endl;           cout.flush();          }             return 0;   }