In which of these situations do the quantities combine to make 0?
A. In the morning, the temperature rises 30 degrees. In the evening, it
falls by 30 degrees.
B. Kathy receives $20 as a gift. She gives half of her $20 to a friend
and keeps the rest for herself.
C. A player in a game earns 4 points for getting an answer right. She
then earns 4 points for making it around the board.
D. A hot air balloon rises 40 feet. It then rises another 40 feet.