What is Calvin's background, and how did he become a U.S. citizen?

a) Calvin is from the United States, and he became a citizen at birth.
b) Calvin is from South Africa and became a U.S. citizen after moving there at the age of 20.
c) Calvin is a native of the United States who later moved to South Africa.
d) Calvin is originally from South Africa, and he is not a U.S. citizen.