Assertion: Amount of organic matter synthesised by producers per unit time and per unit during the process of photosynthesis reffered to as net primary producitity.
Reason: Primary productivity is usually high and sustained throughout the year in temperate areas due to abundance of sunlight.

Answer

D

Explanation

The total amount of organic matter synthesised by producers per unit time and per unit area in the process of photosynthesis is high and sustained throughout the year in tropical areas while it is limited in temperatre areas by cold climate and small growing period. Maximum sunlight is available in tropics whereas poles receive minimu sunlight. Due to this, photosynthesis is maximum and net primary productivity (NPP) is highest `(gt 20 t ha^(-1) year^(-1))` in tropics against average `(8 t ha^(-1)year^(-1))` in temperate forests.