The arms race ment that once the united states builthydrogen bombs,
a. the soviet union built them too.
b. world peace had been assured
c. no other country wanted to build them
d. a new war was about to start

Most likely A.

However, it could also be C.) depending on the context of the question.  If the US was able to create the hydrogen bomb and it was considered a "race" quite like the "race" to put the man on the moon, then it would most likely be C.) since it had already been developed,

However, the soviet union did end up developing the hydrogen bomb after the United States did because of their use of Soviet Spies