单项选择题

The ratio of the work done by the machine ______ the work done on it is called the efficiency of the machine.

A．against
B．with
C．to
D．for

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单项选择题

______ the 1500's ______ the first European explored the coast of California.

A．It was not until/then
B．It is until/that
C．It is not until/when
D．It was not until/that

单项选择题

All flights ______ because of the storm, they decided to take the train.

A．having canceled
B．were canceled
C．having been canceled
D．have been canceled

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