为什么要乘那个C（i，k）？多重集里的rg为啥要挑出来k个？

res = (res + sign * (fac[n - i] * qmi(down, mod - 2, mod) % mod * C(k, i) % mod) + mod) % mod;
AC
res = (res + sign * (fac[n - i] * qmi(down, mod - 2, mod) % mod) + mod) % mod;
通过9/35