单项选择题

The passage mentions that at various points, scientists have believed all of the following to be essential for the nitrogen fixation process EXCEPT

(A) The presence of ammonia
(B) The permeability of cell membranes
(C) The presence of rhizobia
(D) The synthesis of amino acids
(E) Nitrogen-containing molecules like nucleotides

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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.

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

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