问题
单项选择题
The system of derricks and winches has been largely replaced on newer ships by cranes（）it is very efficient.
A.when
B.because
C.unless
D.though
答案
参考答案：B
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The system of derricks and winches has been largely replaced on newer ships by cranes（）it is very efficient.
A.when
B.because
C.unless
D.though
参考答案：B