cas = int(input())

def gcdExtended(a, b):
    # Base Case
    if a == 0 :
        return b,0,1
             
    gcd,x1,y1 = gcdExtended(b%a, a)
     
    # Update x and y using results of recursive
    # call
    x = y1 - (b//a) * x1
    y = x1
     
    return gcd,x,y

def sol(x,y,m,d):
    x = abs(x - y) % m
    g,a,b = gcdExtended(d,m)
    if x % g != 0:
        return -1
    # print(g,a,b,d,m)

    am = m // g
    a1 = a * (x // g)
    a1 = abs(a1) % am

    # print(a1,am)
    return min(a1,am-a1)

    




def run():
    n,m,d = map(int, input().split())
    a = [*map(int, input().split())]

    ans=0
    for i in range(n//2):
        t = sol(a[i], a[n-1-i], m, d)
        if t == -1:
            return 'IMPOSSIBLE'
        ans += t
    return ans
    

for ca in range(cas):
    ans = run()
    print(f'Case #{ca+1}: {ans}')