单项选择题

Researchers warn (against) taking (large numbers of) vitamin A pills, because the tablets contain a form of (the chemical) flint that can be extremely toxic (with high doses).

A. against
B. large numbers of
C. the chemical
D. with high doses
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单项选择题
Helium is not found in the earth’s atmosphere because ______.
A．its moving speed well exceeds 11. 2 km per second
B．it left the atmosphere long ago in search of another planet
C．its moving speed is not great enough to reach the atmosphere
D．it is lighter and cannot be thrown upwards
单项选择题
(While) smoking was associated with an attention of weight gain among black adults, (no such effect) occurred among white men or women, the latter (was) the group (most likely) to smoke for weight control.
A. While
B. no such effect
C. was
D. most likely
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