【单选题】

The passage suggests which of the following about scientists' conception of the function of the amino acid alanine in the rhizobia's nitrogen fixation process
A．(A) They have proposed at least one hypothesis that might account for the function of the immediate conversion of ammonia into alanine.

B．(B) They have established the relationship between alanine and the production of nitrogen-containing molecules beneficial to soybean plants.
C．(C) They continue to believe that ammonia plays an integral role in the symbiosis between rhizobia and soybean plants.
D．(D) They remain uncertain as to how glutamine and glutamate can be produced in the absence of ammonium or ammonia.
E．(E) They have largely rejected the notion that ammonia or ammonium plays any role in nitrogen fixation.