Variation in own words

The act, process, or accident of varying in condition, character, or degree: Prices are subject to variation. A varied form of a melody or theme, especially one of a series of such forms developing the capacities of the subject

Explanation:

my teacher told me this before.

a. because the u.s. was the leading with the most technology, economy, etc. also was teamed with other allies witch boosted the economy and materials to improve technology and use it.

the answer is b your welcome : )