15. Biologists believe that they have found one of the substances that tell individual genes both when to become active and when to remain quiescent in the earliest phases of an embryo’s development.
(A) tell individual genes both when to become active and when to remain
(D) tells individual genes both when to become active or when to remain
这是"one of the substances"，怎么选A呢？？
关键点在“one of the substances that tell...bla bla...”
that从句 + (tell) 想修饰的内容是前面的substances，而不是one of
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另举一例你看看：
One of the glasses is dirty.
如果要对glasses做个限制，应该是：One of the glasses, that are placed on shelf, is dirty
如果改为： One of the glasses that is dirty 。句意就完全不同了，甚至就变成了病句!
此题的关键还是逻辑意思要理解
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