请教prep-ps2-37-----URGENT

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) 

(D) 

(E) 2

如果p是质数,那f(p)应是小于p的质数的个数。请问如何推出 答案 A: p-1

代数字?

p=2 f(p)=1 只有1

p=3 f(p)=2 包括1,2

p=5 f(p)=4 包括1,2,3,4