Which of the following statement(s) is (are) true ?

A) Screening effect $\uparrow$, effective nuclear charge $\downarrow$ thus valence shell electron is loosely bound. Hence I.E.$\downarrow$

            B) Be and Mg has $n{s}^2$ configuration (stable configuration)

            C) Due to lanthanide contraction

            D) $r_{metallic}>r_{\mathrm{cov}alent}$ (covalent bond formation involves the overlapping of                      orbitals)