Choose the answer which lists ionic compounds NaCl, MgS, KCl, and K2S in order of increasing lattice energy (smallest lattice energy listed first).
a) NaCl < MgS < KCl < K2S
b) KCl < K2S < MgS < NaCl
c) K2S < MgS < NaCl < KCl
d) KCl < NaCl < K2S < MgS
e) K2S < KCl < MgS < NaCl