def main():
    n, d = map(int, input().split())
    a = [int(i) for i in input()]

    for c in range(len(a)):
        i = a[c]
        if i < d:
            a.insert(c, d)
            return "".join([str(b) for b in a])

    return "".join([str(b) for b in a])+str(d)


for _ in range(int(input())):
    print(main())