#include<bits/stdc++.h>
using namespace std;
using ll = long long;
void solve(int T){
    string a,b;
    cin>>a>>b;
    int sz=a.size();
    string ans(sz,'a');
    vector<int> ed(sz+1);
    for(int i=sz-1;i>=0;i--){
        if(a[i]!=b[i])  ed[i]=ed[i+1]+1;
        else ed[i]=ed[i+1];
    }
    pair<int,int> sum={0,0};
    for(int i=0;i<sz;i++){
        for(int j=0;j<26;j++){
            pair<int,int> psum=sum;
            char c=(char)(j+'a');
            if(a[i]==c) psum.first++;
            if(b[i]==c) psum.second++;
            if(abs(psum.first-psum.second)<=ed[i+1]){
                ans[i]=c;
                sum=psum;
                break;
            }
        }
    }
    cout<<"Case "<<T<<": "<<ans<<'\n';
}
signed main(){
    ios::sync_with_stdio(0),cin.tie(0);
    int  _ = 1;
    cin >> _;
    int tot=0;
    while (_--) solve(++tot); 
}