import sys
input = sys.stdin.readline
def gcd(a, b):
    while b:
        a, b = b, a % b
    return a

from operator import itemgetter
import math

from functools import lru_cache
@lru_cache(maxsize=None)
def euler_totient(x):
    ANS=x
    
    # 素因数分解
    L=int(math.sqrt(x))

    FACT=dict()

    for i in range(2,L+2):
        while x%i==0:
            FACT[i]=FACT.get(i,0)+1
            x=x//i

    if x!=1:
        FACT[x]=FACT.get(x,0)+1

    # φ(x)=x(1-1/p_1)...(1-1/p_m)という性質を使って計算

    for f in FACT:
        ANS=ANS*(f-1)//f

    return ANS

T=int(input())
for tests in range(T):
    W,N,D=map(int,input().split())
    X=list(map(int,input().split()))

    ANS=0

    for i in range(W//2):
        x=X[i]
        y=X[len(X)-1-i]

        if x==y:
            continue
        else:
            k=(x-y)%N
            l=(y-x)%N

            GCD=gcd(gcd(k,D),N)

            a=D//GCD
            k//=GCD
            l//=GCD
            mod=N//GCD

            if gcd(a,mod)!=1:
                ANS+=1<<60
            else:
                phi=euler_totient(mod)
                sc=k*pow(a,phi-1,mod)%mod
                sc2=l*pow(a,phi-1,mod)%mod

                ANS+=min(sc,sc2)

    if ANS>=(1<<60):
        ANS="IMPOSSIBLE"

    print("Case #"+str(tests+1)+": "+str(ANS))