单项选择题

The objective of the optimal economic operation of power systems is ( ).

A.to
B.to
C.to
D.to
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The distance protection is applied to protect ( ).
A.transformers
B.generators
C.transmission
B.buses
单项选择题
A pop singer has to spend a lot of money （）.

A.for his training to
B.to be unusual
C.to help the poor gain popularity
D.from the public
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