单项选择题

In ancient times, some catastrophic extinction of species occurred ______.

A．sequentially
B．repeatedly
C．naturally
D．intentionally
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单项选择题
Cosmic rays of various kinds come through the air from outer space, but enormous quantities of radiation from the sun are ______ off.
A．avoided
B．excluded
C．screened
D．separated
单项选择题
The professor gave ______ instruction to the whole class so as to make every student understand how to conduct the experiment in the lab.
A．explicit
B．afflictive
C．authoritative
D．oblivious
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