OG12 080 请问定语从句的that可以省略吗？

liang12460

A report by the American Academy for the Advancement of Science has concluded that much of the currently uncontrolled dioxins to which North Americans are exposed comes from the incineration of wastes.
(A) much of the currently uncontrolled dioxins to which North Americans are exposed comes
(B) much of the currently uncontrolled dioxins that North Americans are exposed to come
(C)   much of the dioxins that are currently uncontrolled and that North Americans are exposed to comes
(D) many of the dioxins that are currently uncontrolled and North Americans are exposed to come
(E) many of the currently uncontrolled dioxins to which North Americans are exposed come

选择E。
OG中针对D选项的解释是：
D    That are is wordy; to maintain the parallel in this construction, that would have to be repeated in the clause that North Americans are exposed to.

我确定知道宾语从句的that不可以省略，但是定语从句的that也是一定不可以省略吗？OG12的80题里的D选项里面的解释说不可以省略，是由于平行的原因而不能省略。那么平时，没有平行结构限制时，定语从句的that可以省略吗？

feifan1984

关系代词在从句做宾语，可省略，口语中比较常见，不过GMAT不得而知。不要用他作为判断选项对错吧，像这个题，用平行结构更好判断。

sapphirezhang

定语从句谓语的平行，不需要补出that，相当于共享了句子成分，
不似宾语从句两个名词性从句的平行需要补出从句引导词，如that/whether等。