Which of the following best describes the United States' policy of containment following World War II?

a. The United States would invade the USSR whenever that nation seemed likely to stage a communist takeover.

b. The United States would provide diplomatic assistance only to countries fighting a communist takeover.

c. The United States would only agree go to the United Nations on behalf of nations resisting a communist takeover.

d. The United States would use economic and military measures to prevent the spread of communism.