Which piece of textual evidence best supports the inference that the British had better code breakers than the Russians did?

a The Russian admiralty decided that their British allies could make better use of the codebook than they could, so it was sent to London.
b The codebook was a bonanza for the British code breakers.
c Not only did it contain the columns of code "words"—groups of randomly selected numbers—on which the messages were based
d But it also included a changeable key to the cipher systems used to obscure the coded messages.