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 benefit from reading one more hour is 

The marginal benefit from reading is measured by 

A. the 70 pages you read; the time at the gym you must give up O 

B. the extra 20 pages you read; the time at the gym you are willing to give up

C. the extra 20 pages you read; the time at the gym you must give up 

D. the extra 20 pages you read; the total time you spent reading