the + 单数名词指一类？大全section18-21讨论

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？