请教一个关于as的问题

Until 1850, the speed of signals along nerves was believed to be at least as fast as light does travel.

A.       fast as light does travel

B.       the speed of light

C.      fast as the speed of light

D.      the speed of light is

E.       fast as the speed of light is

Answer : B

根据语感，我觉得这几个答案都不理想，似乎Until 1850, the speed of signals along nerves was believed to be at least as fast as that of light.更好一点。凡是不能理解为什么选择B，sth is believed to be as sth是什么意思？as在这里起什么作用？望诸位NN指点！

Simplify the sentence:

The speed of signal was believed to be as the speed of light.

as: just like

His last album sold half a million copies and we hope this one will be just as (=equally) popular. (Longman)

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LZ   的题是哪里的呢? OG11里的吗? 我看C好些呀?