【单选题】

It can be inferred from the passage that scientists are "perplexed by the means" (line 12) for which of the following reasons
A．(A) They are unsure of how alanine remains separate from the everyday nitrogen metabolism of the plant itself.

B．(B) They have yet to establish what function the alanine serves for the soybean plants.
C．(C) They have thought alanine to exist only in very small quanties in rhizobia.
D．(D) They have long considered the membranes of the soybean plant impermeable to amino acids.
E．(E) They are uncertain how such a rapid export of fixed nitrogen is possible through the soybean membranes.