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單項(xiàng)選擇題

The problem is（）by pollution.

A.caused
B.because
C.due
D.because of

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單項(xiàng)選擇題

The mistake was（）by carelessness of the assistant.

A.due
B.caused
C.because
D.owe

單項(xiàng)選擇題

The volunteers started looking after other children（）parents died of AIDS last year

A.whose
B.that
C.which
D.who

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