单项选择题

If all the continents and mountains were bulldozed flat, the earth __ covered by water.

A．would be
B．would have been
C．would had been
D． would been
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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
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
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