B.are are kind of vegetable
Potatoes are a kind of vegetable.
马铃薯是一种蔬菜.
potatoes是复数,显然后面不能是a vegetable,从而可以否定A
is a 也与主语s不一致,否定C
D.虽然与主语一致,但它只是一种蔬菜,因此不能用vegetable（这里是指种类）