Assertion (A) The elements having $1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2\text{ and }1{\mathrm{s}}^2,2{\mathrm{s}}^2$ configuration belong to same period.
Reason (R) Both have same number of outermost shells.

A is correct; R is incorrect.

R is correct; A is incorrect.

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

Both A and R are correct; and R is not the correct explanation of A.