Definition: You are given a question on US foreign policy. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: According to Realists, what accounts for the onset of the Cold War?
(A)Ideological differences (B)A power vacuum (C)The expansionist nature of the Soviet Union (D)Both b and c

D