单项选择题

The gas-oil contact or gas-water contact_____ the lower limit of producible gas.

A． is
B．are
C． were
D．has been
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单项选择题
For the creatures to become oil, it was necessary that they ______ between layers of rock for an enormous length of time.
A． been imprisoned
B．shall be imprison
C． should be imprison
D． will be imprisone
单项选择题
The assumption _____ human cloning rests is that all genetic cells contain exact copies of the original set o9f genetic instructions.
A． on which
B．on that
C．at which
D． which
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