[求助]一道prep数学题

The function f is defined for all positive integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number then f(p)=
A.p-1
B.p-2
C.(p+1)/2
D.(p-1)/2
E.2

答案：A

不知怎么做的，恳求各位大侠帮帮忙～～谢谢啦！！