[求助] SC 885 - 177

audrey_zhou

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

Why is the answer C? 两个从句之间并无连词(believed to ..., that they ...)

gemj

believed to originate from a small area on their foreheads, 是插入成分。