Microeconomics 18th Edition by Campbell McConnell, Stanley Brue, Sean Flynn

Exercise 16

Marginal product in graph (b) is zero when:
a. average product in graph (b) stops rising.
b. the slope of the marginal-product curve in graph (b) is zero.
c. total product in graph (a) begins to rise at a diminishing rate.
d. the slope of the total-product curve in graph (a) is zero.