int main() {

    int t;
    cin>>t;
    int ppp=t;
    while(t--){
        int n;
        cin>>n;
        vector<pair<int,int>> v;
        int arr[n];
        int dep[n];
        for(int i=0;i<n;i++){
            int a,d;
            cin>>a>>d;
            arr[i]=a;
            dep[i]=d;
            v.push_back(make_pair(a,d));
        }
        int ma=0;
        int w[10000]={0};
        for(int j=0;j<n;j++){
            int a,b;
            a = v[j].first;
            b = v[j].second;
            for(int i=a;i<b;i++){
                w[i]++;
                ma = max(ma,w[i]);
            }

            }

        if(ma>2){
            cout<<"Case #"<<ppp-t<<": IMPOSSIBLE\n";
            continue;
        }

        string s;

        int work[10000]={0};
        for(int j=0;j<n;j++){
            int a,b;
            a = v[j].first;
            b = v[j].second;
            int m=0;
            for(int i=a;i<b;i++){
                m = max(work[i],m);
            }

            if(m%2==0){
                s+='C';
                for(int i=a;i<b;i++){
                    work[i] = 1;
                }
            }
            if(m==1){
                s+='J';
                for(int i=a;i<b;i++){
                    work[i] = 2;
                }
            }


        }

         cout<<"Case #"<<ppp-t<<": "<<s<<"\n";

       // cout<<"Case #"<<ppp-t<<": ";

    }


    return 0;
}