1) n has exactly two positive factors.
2) the difference of any two distinct positive factors of n is odd.
這題他的答案是B. 解釋是"If n >2 and n is even, then 2 and n are factors of n whose difference is even. Thus, no integer greater than 2 satisfies statement (2), but 2 itself does since 1 and 2 are the only positive factors of 2.
可是 if n =8, then 1 and 8 are factors of n whose difference is odd. 所以他的解釋好像不是很正確. Statement (2) is not sufficient to answer? 麻煩幫我看一下
謝謝!!
