求教今年sub math官方practice book最后一题

Let Z17 be the ring of integers modulo 17, and let Z17* be the group of units of Z17 under multiplication. Which of the following are generators of Z17*?

I.5

II.8

III.16

(A) None (B) I only (C) II only (D) III only (E) I, II, and III

答案是B...为啥不是E啊，不是和17互素的数都可以吗