feifei 89 请指点~~

？？？89. Rhizobium bacteria living in the roots of bean plants or other legumes produce fixed nitrogen, which is one of the essential plant nutrients and which for non-legume crops, such as wheat, normally must be supplied by applications of nitrogen-based fertilizer. So if biotechnology succeeds in producing wheat strains whose roots will play host to Rhizobium bacteria, the need for artificial fertilizers will be reduced.

The argument above makes which one of the following assumptions?

(A) Biotechnology should be directed toward producing plants that do not require artificial fertilize.

(B)  Fixed nitrogen is currently the only soil nutrient that must be supplied by artificial fertilizer for growing wheat crops.

(C) There are no naturally occurring strains of wheat or other grasses that have Rhizobium bacteria living in their roots.

(D) Legumes are currently the only corps that produce their own supply of fixed nitrogen.

(E) Rhizobium bacteria living in the roots of wheat would produce fixed nitrogen.

像这样的题目，只有A有not，但又不是正确答案，做assumption 的时候碰到这样的题目该怎么办呢？是不是assumption题目用取非的方法不一定行得通？

各位同胞指点一下~~~想得有点头疼了~~~