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

Ans: a