单项选择题

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.

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单项选择题

Despite the______, Alzheimer’s researchers are still______about the prospects for immunization, though most agree that more results are needed from humans for us to truly understand immunization’s effects.

(A) advances… thrilled
(B) obstacles … uncertain
(C) setback… optimistic
(D) publicity … divided
(E) pessimism … unconvinced

单项选择题

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