For n independent flips of a fair coin the probability of k heads (0 <= k <= n) is
a) (n! / ((n-k)!k!)) * (1/2^n)
b) (n! / ((n-k)!k!)) * (1/2)
c) (n-k)! / (n!k!) * (1/2^n)
d) (n! / (n-k)!) * (1/2^n)