a structure issue？？

gerry0601

12.The chief sources of B12,a water-soluble vitamin ____ stored in the body, include meat, milk and eggs.

(A) is not

(B) that is not

(C) not that is

(D) that not

answer is B

but i think is A

anybody can help m??  thank u!

gerry0601

自己想了会知道了！！

还是谢谢了