Which of the Following Transactions Adds to U

Which of the following transactions adds to U.S.GDP for 2015?

A) In 2015,Ann sells a car that she bought in 2011 to Bill for $7,000.
B) An American management consultant works in Canada during the summer of 2015 and earns the equivalent of $40,000 during that time.
C) When Ken and Kim were both single,they lived in separate apartments and each paid $800 in rent.Ken and Kim got married in 2015 and they bought a previously unoccupied house that,according to reliable estimates,could be rented for $1,700 per month.
D) None of the above transactions adds to U.S.GDP for 2015.