Diminishing marginal returns occur when: A) units of a variable input

Diminishing marginal returns occur when:

A) units of a variable input are added to a fixed input and total product falls.
B) units of a variable input are added to a fixed input and marginal product falls.
C) the size of the plant is increased in the long run.
D) the quantity of the fixed input is increased and returns to the variable input fall.

ANSWER

B