在三相交流电路中所谓三相负载对称是指()、()和()。
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Large numbers of drivers in China now depend _____BeiDou for their routing directions.
A. on
B. with
C. for
D. to
We were unable to offer that service _____we hadn't got sufficient personnel to cope with it.
A. although
B. till
C. unless
D. because