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 was a competition between the US and the Soviet Union during the Cold War. When one side developed a new weapon (like hydrogen bombs), the other side felt the need to match it to maintain a balance of power. World peace was not assured as the arms race increased tensions. Other countries (like the Soviet Union in this context) did want to build such weapons. And while tensions were high, it doesn't mean a new world war was immediately about to start just because the US built hydrogen bombs. The key concept is the reciprocal nature of the arms - race.

Answer:

the Soviet Union built them too.