#include <bits/stdc++.h>
#define x first
#define y second
#define all(v) v.begin() , v.end()
#define ios ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
#define int long long

using namespace std;

typedef long long LL;
typedef pair<int , int> PII;

const int N = 3e5 + 5;
const int mod = 998244353;
const int INF = 0x3f3f3f3f;

int suf[N];
string s , t;

int cal(char c , int pos){
    int a = 0;
    if(c == s[pos])a --;
    if(c == t[pos])a ++;
    return a;
}

void solve(int x){
    cout << "Case " << x << ": ";
    cin >> s >> t;
    int n = s.size();
    for(int i = n - 1 ; i >= 0 ; i --){
        if(s[i] != t[i])suf[i] = 1;
    }
    for(int i = n - 2 ; i >= 0 ; i --)suf[i] += suf[i + 1];
    suf[n] = 0;
    vector<char> q;
    int ss = 0;
    for(int i = 0 ; i < n ; i ++){
        for(int j = 0 ; j < 26 ; j ++){
            if(suf[i + 1] >= abs(ss + cal('a' + j , i))){
                q.push_back('a' + j);
                ss += cal('a' + j , i);
                break;
            }
        }
    }
    for(auto c : q)cout << c;
    cout << endl;
    for(int i = 0 ; i <= n ; i ++)suf[i] = 0;
}

signed main(){
    ios;
    int T = 1;
    cin >> T;
    int tot = T;
    while(T --){
        solve(tot - T);
    }
    return 0;
}