单项选择题

Ecology, __ the relationships between organisms and their environments, is also important in petroleum geology.

A．the study of
B．it study
C．that study
D．studying
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单项选择题
Has he come back .... Yes, he __ back for three days.
A．has come
B．had come
C． is
D．has been
单项选择题
A biologist does not merely describe organisms, but tries to learn __ act as they do.
A．what to cause them
B．what cause to them
C．what causes them to
D．causes them to what
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