OG12 62 关于that，求牛牛总结~~！

og12 - 6262 . When Congress reconvenes, some newly electedmembers from rural states will try and
establish tighterrestrictions for the amount of grain farmers are to beallowed to grow and to encourage
more aggressivesales of United States farm products overseas.

(A) and establish tighter restrictions for the amountof grain farmers are to be allowed to grow andto encourage
(B) and establish tighter restrictions on the amountof grain able to be grown by farmers andencouraging
(C) establishing tighter restrictions for the amountof grain farmers are allowed to growand toencourage
(D) to establish tighter restrictions on the amount ofgrain capable of being grown by farmers andencouraging
(E) to establish tighter restrictions on the amount ofgrain farmers will be allowed to grow and toencourage

此题选E，答案木有问题，通过平行可以判断出来，我的问题是，在the amount of grain（that）farmers are...中的that为什么可以省略？看了前面的帖子都没有说清楚什么时候that 可以省略什么时候不能省略求教各位，非常感谢