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Which of the following statements is correct for the periodic classification of elements?

A) Atomic size gradually increases from left to right in a period of representative elements.

B) Across a transition series, atomic size gradually but somewhat irregularly decreases and then increases at the end of the series.

C)  Electron gain enthalpies of third period elements, sulphur and chlorine are less negative than those of oxygen and fluorine of second period in respective groups.

D)  lonisation   potential   gradually  but irregularly decreases across a period in representative elements.