Mario Sees Popcorn and Pretzels as Perfect Substitutes

Mario sees popcorn and pretzels as perfect substitutes.He is always willing to substitute 3 cups of popcorn for 1 cup of pretzels.His indifference curves for popcorn and pretzels are and at the optimal consumption bundle his marginal rate of substitution of popcorn in place of pretzels is
________.

A) right angles; 3
B) right angles; 0.33
C) straight lines;
D) 3 straight lines; 1/3