Assertion (A): Avogadro number is denoted by ${\text{N}}_{\text{A}}$

 Reason (R): Gram molecular weight of oxygen is 32 a.m.u

A is correct and R is incorrect.

Both A and R are correct and R is the correct explanation of A.

A is incorrect and R is correct.

Both A and R are correct but R is not correct explanation of A.