The marginal product of labor (MPL) is defined as: 

Select one: 

A. The extra output produced by employing one more unit of labor while allowing other inputs to vary 

B. A firm's total output divided by total labor input 

C. The extra output produced by employing one more unit of labor while holding other inputs constant. 

 D. The extra output produced by employing one more unit of capital while holding labor input constant X