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
(No wonder) that (man’s) great dream has been someday to control the weather. The first step toward control is, of course, knowledge, and scientists have been (hard at work) for years trying to (keep track with) the weather.
A. No wonder
B. man’s
C. hard at work
D. keep track with
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