Which of the Following Sets Are Isoelectronic (I

Multiple Choice

Which of the following sets are isoelectronic (i.e., have the same number of electrons) ?
i. Br^-, Kr, Sr²⁺
Ii. C, N^-, O^2-
Iii. Mg²⁺, Ca²⁺, Sr²⁺
Iv. O^2-, O, O²⁺
V. Ag⁺, Cd²⁺, Pd

A) i and ii
B) i and v
C) i, iii and iv
D) ii, iii and v
E) i, iii iv and v