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177. Believed to originate from a small area on their foreheads, elephants emit low-frequency sounds that may be used as a secret language to communicate with other members of the herd.

(A) Believed to originate from a small area on their foreheads, elephants emit low-frequency sounds that may be used

(B) Elephants emit low-frequency sounds that are believed to originate from a small area on their foreheads, and they may use this

(C) Elephants emit low-frequency sounds, believed to originate from a small area on their foreheads, that they may use

(D) Originating, it is believed, from a small area on their foreheads; elephants emit low-frequency sounds they may use（C）

(E) Originating, it is believed, from a small area on their foreheads, low-frequency sounds are emitted by elephants that may be used

这道题好像答案都不太对，C里的that 是什么啊，如果是sounds为什么用单数形式呢？而且又是就近指代foreheads，所以不对吧。

c中的that是定语从句的引导词,Elephants emit low-frequency sounds, (sounds are believed to originate from a small area on their foreheads), that they may use

其中括号部分是插入语,实际结构是Elephants emit low-frequency sounds that they may use.上文中黄颜色部分是插入语的省略部分.that在此处是定语从句的引导词,在从句中做宾语 they may use that根据语法要求可省略