[求助]亲爱的们，帮忙看两道数学题咯~~

7. In the expression (a/b)/c, a, b, and c are
different numbers and each is one of the
numbers 2, 3, or 5. What is the least
possible value of the expression?
(A)1/30
(B)2/15
(C)1/6
(D)3/10
(E)5/6

本人觉得应该是A咯，答案是B。。。

20. If y ≠ 3 and3x/y is a prime integer
greater than 2, which of the following
must be true?
Ⅰ. x = y
Ⅱ. y = 1
Ⅲ. x and y are prime integers.
(A) None
(B) Ⅰ only
(C) Ⅱonly
(D) Ⅲonly
(E) Ⅰand Ⅲ

请麻烦说明过程，谢谢~~

第一题选B，第二个选A。。。

还是不太明白第一个B。。。