#include <stdio.h>

typedef struct event {
    char person;
    int a[2];
} event;

int main(void) {
    int x;
    scanf("%i", &x);
    for (int i = 0, y = x; i < y; i++) {
        int rows;
        scanf("%i", &rows);
        int ptc;
        event arr[rows];
        for (int j = 0; j < rows; j++) {
            int cfrequency = 0;
            int cokfrequency = 0;
            int jfrequency = 0;
            int jokfrequency = 0;
            scanf("%i %i", &arr[j].a[0], &arr[j].a[1]);
            if (j == 0) {
                arr[0].person = 'C';
            }
            else if (j == 1) {
                arr[1].person = 'J';
            }
            else {
                ptc = j - 1;
                while (ptc >= 0) {
                    if (arr[ptc].person == 'C') {
                        cfrequency++;
                        if (arr[j].a[0] >= arr[ptc].a[1] || arr[j].a[1] <= arr[ptc].a[0]) {
                            cokfrequency++;
                        }
                    }
                    else {
                        jfrequency++;
                        if (arr[j].a[0] >= arr[ptc].a[1] || arr[j].a[1] <= arr[ptc].a[0]) {
                            jokfrequency++;
                        }
                    }
                    ptc--;
                }
                if (cfrequency == cokfrequency) {
                    arr[j].person = 'C';
                }
                else if (jfrequency == jokfrequency) {
                    arr[j].person = 'J';
                }
                else {
                    arr[0].person = 'A';
                    ptc = 0;
                }
            }
        }
        if (arr[0].person == 'A')
        {
            printf("Case #%i: IMPOSSIBLE", i + 1);
        }
        else
        {
            printf("Case #%i: ", i + 1);
            for (int b = 0; b < rows; b++)
            {
                printf("%c", arr[b].person);
            }
        }
    printf("\n");
    }
}