If AIDS Was Present in Africa in the 1950's, What

If AIDS was present in Africa in the 1950's, what is the most likely reason why did it not appear in the United States until the 1970's?

A) It wasn't until the 1970's that large-scale humanitarian aid (including susceptible workers) began to move into Africa from Western countries.
B) Travel to and from Africa was relatively rare in the 1950's. This would mean that it would have been very improbable that an infected individual would have come to the US from Africa and begun spreading the illness.
C) The virus needed a longer period of time to mutate in order to become more virulent and spread more easily. Even in the 1950's in Africa, it most likely wasn't able to be transmitted very easily from person to person (yet) .
D) Sexual practices and behaviors were different in the 1950's than they were in the 1970's. This led to less sexual intercourse outside of monogamous married relationships, and less likelihood of spread of the illness between people.