George's shoe store buys and sells shoes in pairs. If George's isoquants are drawn with quantity of right shoes on the vertical axis and quantity of left shoes on the horizontal axis, what do they look like? They are: a. curves that get flatter as the number of left shoes increases. b. curves that get steeper as the number of left shoes increases. c. straight lines that have a negative slope. d. L-shaped. e. horizontal straight lines.