Assertion (A) : Atomic mass of Mg is 24 u

Reason (R) : An atom of Mg is 24 times heavier than $\frac{1}{12}$ ^th mass of an atom of a C-12 isotope.

Both (A) and (R) are true and (R) is correct explanation of (A)

Both (A) and (R) are true but (R) is not the correct explanation of (A)

(A) is true but (R) is false

Both (A) and (R) are false