OG4

The only way for growers to salvage frozen citrus is to process them quickly into juice concentrate before they rot when warmer weather returns.

(A)   to process them quickly into juice concentrate before they rot when warmer weather returns

(B)   if they are quickly processed into juice concen­trate before warmer weather returns to rot them

(C)   for them to be processed quickly into juice concentrate before the fruit rots when warmer weather returns

(D)   if the fruit is quickly processed into juice concen­trate before they rot when warmer weather returns

(E)    to have it quickly processed into juice concentrate before warmer weather returns and rots the fruit

E also expresses the cause-and-effect relationship between the return of warmer weather and the rotting of the fruit; A, C, and D merely describe these events as contemporaneous.

因为 (E)  to have it quickly processed into juice concentrate before warmer weather returns and rots the fruit

returns and rots表示的是两个一前一后的动作，自然也表现出了他们的因果关系，要warm weather 先returns然后再rots the fruit。

例如c中，the fruit rots when warmer weather returns 水果会在warmer weather回来的时候坏。只是表明水果在什么时间坏，可以碰巧是天气变热的时候坏，无法直接的体现出它们之间的因果关系。