Bob tells his friends, "You may call me Bobby, you may call me Zimmy." Plotting the name Bobby on the vertical axis and the name Zimmy on the horizontal axis, what is the shape of each of Bob's indifference curves over the two names? (Think of the quantities on each of the axes as the number of times someone calls him one name or the other.)

a. Bob's indifference curves are strictly convex to the origin.

b. Bob's indifference curves are strictly concave to the origin.

c. Bob's indifference curves are straight lines with slope -1.

d. Bob's indifference curves are straight lines with slope +1.