想请问一下这题文法题
20. Ecologists often study issues ____ the effects of human activities upon
populations and ecosystems.
(A) whom relate to
(B) which relates to
(C) that relate to
(D) that they relate to
请问D选项 为何不能解释成名词子句，they= issues这样
明明知道很怪但说不出论点，还请各位赐教，非常感谢!

they (ecologists) relate the issues to the effects...

所以请问文法上是错在they不能指定是issues这样吗?

if you insist "that" 就是issues, then it should be ~that are related to the effects... 先试着理解relate 这个字的用法。勿跟 the way that... the reason that...混谈它们2者惯用法不像这句有想被关联的先行词

好的~谢谢指导，当名词子句来看语意也不大通我觉得