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
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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
单项选择题
Between 1890 and 1970 the monetary costs of supplying energy _____ more or less constant or declined.
A．to stay
B．was stayed
C．stayed
D．has stayed
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