Sarah Jones believes that Coke and Pepsi are perfect substitutes. That is, she is equally happy with a Coke or a Pepsi and cannot tell the difference between them. Her indifference curves:
a. are linear, with slope equal to –1.
b. are upward-sloping.
c. are linear, with slope equal to 1.
d. cross where she consumes equal amounts of Coke and Pepsi.
e. cannot be defined, since she likes both equally well.