马上要考了，请各位帮忙看看这个问题

59，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

谁能告诉我这个as在这里是做什么用的？

as指代前面主句

例:Although it claims to delve into political issues, television can be superficial, as when the three major networks each broadcast exactly the same statement from a political candidate.