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:

An arms race is a competition between nations to develop and accumulate more powerful weapons. During the Cold War, the U.S. and Soviet Union engaged in this, so when the U.S. built hydrogen bombs, the Soviet Union followed to maintain military parity. The other options are incorrect: an arms race does not assure peace, other countries did pursue nuclear weapons, and a direct world war did not start immediately after the H-bomb development.

Answer:

the Soviet Union built them too