Assertion a : to Estimate the Rainfall Over a Catchment

Assertion A : To estimate the rainfall over a catchment, the number of raingauges required per unit area is large for hilly areas.Reason R : Rainfall gradient is steep.Select your correct answer according to the coding system given below :

A) Both A and R are true and R is the correct explanation of A
B) Both A and R are true but R is not the correct explanation of A
C) A is true but R is false
D) A is false but R is true