请教GWD-11-26

GWD-11-26

On Earth, among the surest indications of sunspot cycles are believed to be the rate that trees grow, as seen in the rings visible in the cross sections of their trunks.

A.    On Earth, among the surest indications of sunspot cycles are believed to be the rate that trees grow

B.    On Earth, among the surest indications of sunspot cycles are, it is believed, the rate of tree growth

C.    On Earth, the rate at which trees grow is believed to be among the surest indications of sunspot cycles

D.    Among the surest indications on Earth of sunspot cycles, believed to be the tree growth rate

E.     Among the surest indications on Earth of sunspot cycles is believed to be the rate at which trees grow

此题答案选E

不明白为什么cycles用单数is

among 在句首要倒装！

正常的语序应该是，the rate at which.....is believed to be among the surest indication on earth of sunspot cycles .

所以要用is。。。。

多谢了！

请问为什么不能选C？