Why Was the Planned 1960 Summit Meeting in Paris Between

Why was the planned 1960 summit meeting in Paris between Eisenhower and Khrushchev canceled?

A) An attempt by dissenters in the Soviet Politburo to depose Khrushchev posed a threat to his power.
B) It appeared that the meeting might benefit the Democrats in the upcoming presidential election.
C) The United States refused to apologize after an American spy plane was shot down over the Soviet Union.
D) Conservative Republicans bitterly protested Eisenhower's willingness to meet with Khrushchev.