#include <iostream> //for taking input as in cstdio
#include <string> //for reading strings directly
#include <vector> //just an array
using namespace std;
int main() {
int t;
cin>>t;
for(int i=0;i<t;++i) {
string a,b;
cin>>a>>b;
vector< char > V(a.size()); // a.size() is length of both strings
for(int j=0;j<a.size();++j) {
if(a[j]=='W' && b[j]=='W')
V[j]='B';
if(a[j]=='B' && b[j]=='B')
V[j]='W';
if((a[j]=='B' && b[j]=='W')||(a[j]=='W' && b[j]=='B'))
V[j]='B';
}
for(int j=0;j<a.size();++j)
cout<<V[j];
cout<<endl;
}
return 0;
}
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