Question

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

Explanation:

The arms race during the Cold War involved the United States and the Soviet Union competing in developing weapons. When the US built hydrogen bombs, the Soviet Union, in the context of the arms race, also developed them to maintain parity. The other options are incorrect: an arms race doesn't assure peace, other countries (like the Soviet Union) did want to build them to compete, and while tensions were high, the arms race was more about mutual deterrence rather than an immediate new world war starting just because one built hydrogen bombs.

Answer:

A. the Soviet Union built them too.