Assertion: Ionisation potential across the period is `NaltAlltMgltSi`.
Reason : Ionisation potential decreases with decrease in atomic size.
A. If both assertion and reason are true and the reason is the correct explanation of the assertion
B. If both assertion and reason are true but reason is not a correct explanation of the assertion
C. If assertion is true but reason is false
D. If the assertion and reason both are false