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