Section 18-21. In the face of widespread concern about environmental waste, compact disc manufacturers are attempting to find a replacement for the disposable plastic box in which they package their product.
(A) the disposable plastic box in which they
(B) the disposable plastic box where they
(C) a disposable plastic box in which to
(D) disposable plastic boxes inside which they（A）
(E) the disposable plastic boxes in which to
manufacturers是复数，但A中box用了单数，后面their product也用了单数，为什么？
那么E错在哪里呢？
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