Assertion `(A)`: Electrolysis of hard water produces `O_2` and `D_2`.
Reason `(R)` :Ordinary hydrogen is called protium.
A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.