Every week, Bob runs for two hours, and his grade on each math test is 80 percent.
Last week, after running for two hours, Bob considered running for another hour. He decided to run for another hour and cut his study time by one hour.
But last week, his math grade fell to 60 percent.
Did Bob make his decision on the margin?
A. Bob's decision was not made at the margin because the marginal cost of a lower grade always exceeds its marginal benefit.
B. Bob made his decision at the margin because he considered the benefit and cost of one additional hour of running.
C. Bob's decision was not made at the margin because he could have chosen not to go running but didn't.
D. Bob's decision was not made at the margin because he didn't compare the total benefit and total cost of a second hour of running.
E. Bob's decision was made at the margin because his grade fell by only 20 percentage points.
