Was the idea that if vietnam became communist, all of se asia would as well.

Explanation:

Domino theory alleged that the spread of communism in Korea and Vietnam would result in other countries in Asia falling like dominoes to the influence of communism.

Domino theory was related also to the US foreign policy of contaiment -- seeking to contain communism where it was and not let it spread.

For more information and illustration, see this answer I posted on -

the correct answer is b, "fertility rates are increasing, so populations will grow more quickly in the future."