单项选择题

Tests have proved that caffeine affects the body by increasing the heart rate and rhythm, which ______affects the circulatory system.

A．in turn
B．in return
C．turn back
D．as a result
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单项选择题
He said it was impossible for ______ a mistake in a computer's calculation, so you can rely on that.
A．there being
B．there would be
C．there to be
D．there was
单项选择题
Speciation, ______, results when an animal population becomes isolated by some factor, usually geographic.
A．form biological species
B．biological species are formed
C．which forming biological species
D．the formation of biological species
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