The "domino theory" during the post-world war ii era held that

the youth of the united states were quickly losing their moral foundation.
if one country were to fall to communist control, countries around it would soon fall.
once humans established a base on the moon, exploration of the planets would quickly follow.
if one southern state would repeal its segregation laws, states around it would soon do the same.

B) if one country were to fall to communist control, countries around it would soon fall.

Explanation:

The "Domino Theory" held that if one country were to fall to communist control, countries around it would soon fall. This led to the U.S. political and military philosophy of "containment" of communist to those countries already affected.

the us constitution is a living document, meaning that it can be changed over time as the country changes.

study of past cultures