What Did the United States Believe Was the Key to Containing

What did the United States believe was the key to containing communism in the countries Stalin already controlled?

A) Keeping a significant military force in all other European countries.
B) Making sure that West Germany was economically successful.
C) Gaining control of East Berlin through clandestine means.
D) Convincing Europe's neutral countries that joining the fight against communism was necessary.
E) Getting rid of Stalin, by assassination if necessary.