21. In the periodic table, five elements A, B, C, D, and E with their atomic numbers inan increasing order of A ＜B ＜C＜D ＜E ＜ 20. B and C are adjacent to each other.B and E exist in the form of diatomic molecules under normal conditions. A reactswith B, C and E to give Q, and Z, respectively, and both and Xare10-electron compounds, while Z is an 18-electron compound. The number ofoutermost electrons in the atom of D equals the number of electron shells. Giventhat Q +Z- M, which of the following statements is correct?
(A)ionic radius: D ＞ C＞ B
(B)Z is an ionic compound.
(C)M is composed of non-metallic elements and contains only covalent bonds.
(D)D can be in the second period of group IIA in the periodic table.
(E)Three types of 10-electron species can be formed by reacting A and C.