#include<iostream>
#include<cstdio>
#include<cstring>
#define I inline
using namespace std;

const int N = 10010;
char a[N],b[N],ans[N];
int mn[N];
int n,d[N];

I int ab(int x) {
	if(x<0) return -x;
	return x; 
}
I void solve(int T) {
	scanf("%s",a+1);
	scanf("%s",b+1);
	n=strlen(a+1);
	for(int i=1;i<=n;++i) {
		for(int j=1;j<=26;++j) {
			if(j+'a'-1!=a[i]&&j+'a'-1!=b[i]) {
				mn[i]=j;
				break;
			}
		} 
	} 
	for(int i=1;i<=n+1;++i) d[i]=0;
	for(int i=n;i>=1;--i) {
		if(a[i]!=b[i]) d[i]++;
		d[i]+=d[i+1];
	}
	int s1=0,s2=0,cv1=0,cv2=0;
	for(int i=1;i<=n;++i) {
		if(a[i]==b[i]) {
			ans[i]='a';
			continue;
		}
		int ns1=s1,ns2=s2,mk=26;
		
		if(ab(s2+1-s1)<=d[i+1]) {
			mk=min(mk,a[i]-'a'+1);
		} 
		if(ab(s1+1-s2)<=d[i+1]) {
			mk=min(mk,b[i]-'a'+1);
		}
		if(ab(s1-s2)<=d[i+1]) {
			mk=min(mk,mn[i]);
		}
		ans[i]=mk+'a'-1;
		if(mk==a[i]-'a'+1) {
			s2++;
		} else if(mk==b[i]-'a'+1) {
			s1++;
		}
	}
	cout<<"Case "<<T<<": ";
	for(int i=1;i<=n;++i) cout<<ans[i]; cout<<endl;
} 
int main()
{
	int T; cin>>T;
	for(int i=1;i<=T;++i) solve(i);
}