In one day, Sue can change the oil on 20 cars or the tires on 20 cars. In one day, Fred can change the oil on 20 cars or the tires on 10 cars. Sue and Fred can gain from trade if Sue changes the ________ and Fred changes the ________:
A) tires; tires
B) tires; oil
C) oil; oil
D) oil; tires
Answer: B