Which Statement Defines Net Primary Productivity

Multiple Choice

Which statement defines net primary productivity? 

A)  It is the rate at which producers use chemical energy through respiration. 
B)  It is the rate of photosynthesis plus the rate of respiration. 
C)  It is the net rate at which producers convert sugar into biomass. 
D)  It is the rate at which producers manufacture chemical energy through photosynthesis.