#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <list>
using namespace std;
using ll = long long;
ll gcd(ll a, ll b)
{
    if (b == 0)
        return a;
    return gcd(b, a % b);
}
int T = 0;
void EachT(int p)
{
    cout << "Case " << p << ": ";
    string a, b, c;
    cin >> a >> b;
    ll dis = 0, now = 0;
    for (int i = 0;i < a.length();i++)
    {
        dis += (a[i] != b[i]);
    }
    for (int i = 0;i < a.length();i++)
    {
        if (a[i] == b[i])
        {
            c.push_back('a');
        }
        else
        {
            dis--;
            if (a[i] != 'a' && b[i] != 'a')
            {
                if (dis >= abs(now))
                    c.push_back('a');
                else
                {
                    if (now > 0)
                    {
                        c.push_back(a[i]);
                        now--;
                    }
                    else
                    {
                        c.push_back(b[i]);
                        now++;
                    }
                }
            }
            else if (a[i] == 'a')
            {
                if (abs(now - 1) <= dis)
                {
                    c.push_back('a');
                    now--;
                }
                else if (b[i] != 'b')
                {
                    if(abs(now)<=dis)
                        c.push_back('b');
                    else
                    {
                        c.push_back(b[i]);
                        now++;
                    }
                }
                else
                {
                    if (abs(now + 1) <= dis)
                    {
                        c.push_back('b');
                        now++;
                    }
                    else
                    {
                        c.push_back('c');
                    }
                }
            }
            else
            {
                if (abs(now + 1) <= dis)
                {
                    c.push_back('a');
                    now++;
                }
                else if (a[i] != 'b')
                {
                    if (abs(now) <= dis)
                        c.push_back('b');
                    else
                    {
                        c.push_back(a[i]);
                        now--;
                    }
                }
                else
                {
                    if (abs(now - 1) <= dis)
                    {
                        c.push_back('b');
                        now--;
                    }
                    else
                    {
                        c.push_back('c');
                    }
                }
            }
        }
    }
    cout << c << endl;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int t;
    cin >> t;
    ll num = 0;
    while (t--)
    {
        EachT(++num);
    }
    return 0;
}