Assertion (A) `:` pH of aqueous solutin decreases when NaCN is added to it .
Reason (R ) `:` NaCN provides a common ion to HCN.
A. Both A and R true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true