When you spend two hours reading, you can read 50 pages. And if you spend three hours reading, you can read 70 pages. To read for the extra hour, you give up working out at the gym for that hour. The marginal reading rate (pages per hour) is:
a) 10 pages per hour.
b) 20 pages per hour.
c) 30 pages per hour.
d) 40 pages per hour.