求助prep上的题目

dpzixuan89

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

我怎么算了是C    答案是A   是不是我题目没看明白？？？ 求助
谢谢！！！

ejason

好题。

题目有点绕： f（p）是求小于与任意质数P，且与P互质的的正数的个数。 P-1 是正确答案