#include
#include <bits/stdc++.h>
#include<string.h>
#include
using namespace std;
int main ()
{
int t;
cin>>t;
while(t–)
{
string s1,s2;
cin>>s1>>s2;
string s3;
for(int i=0;i<s1.length();i++)
{
s3[i]=abs(s2[i]-s1[i]);
cout<<s3[i];
}
}
}