Marc has an income of $20 per week. Root beer costs $5 a can and CDs cost $10 each. The figure, below, illustrates his preferences. a. What are the quantities of root beer and CDs that Marc buys? b. What is Marc's marginal rate of substitution of CDs for root beer at the point at which he consumes? Root beer (bottles per month) 3 2 O 2 3 CDs (number per month)