Coke and Pepsi Are Perfect Substitutes for Mr

Coke and Pepsi are perfect substitutes for Mr. Drinker and the slope of his indifference curves is - 1. One day he bought 2 cans of Coke and 20 cans of Pepsi. (The cans of both drinks are the same size.)

A) Coke is less expensive than Pepsi.
B) Coke is more expensive than Pepsi.
C) Coke and Pepsi cost the same.
D) Mr. Drinker prefers Pepsi to Coke.
E) None of the above.