单项选择题

It can be inferred that "iodine, fluoride, calcium..." (Line 3, Par

A．2) means _________.A. substances which influence human healthB. harmful substances in the water supplyC. substances which help provide breeding places for pestsD. substances contributory to the height and strength of a person
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单项选择题
According to the passage, research into the influence of geography and climate should _________.
A．focus on unknown aspects
B．be pursued on a larger scale
C．be carried out among remote tribes
D．go ahead in depth
单项选择题
The author mentioned the studies in this passage to suggest that _________.
A．we should make better generalizations about types of people in the world
B．we should find better water supplies in some dry areas
C．we should gain insight into the influence of geographical environment on man
D．we should cultivate a favorable psychology and health in our own environment
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