21. In the periodic table, five elements A, B, C, D, and E with their atomic numbers in
an 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 reacts
with B, C and E to give Q, and Z, respectively, and both and X
are10-electron compounds, while Z is an 18-electron compound. The number of
outermost electrons in the atom of D equals the number of electron shells. Given
that 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.
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