A materials scientist screens four rock-salt ionic solids as simplified models for selecting a refractory ceramic. The ionic radii are shown in the table.

Solid	Cation radius, \(r_+\) (\(\text{pm}\))	Anion radius, \(r_-\) (\(\text{pm}\))
\(\text{MgO}\)	\(72\)	\(140\)
\(\text{CaO}\)	\(100\)	\(140\)
\(\text{NaF}\)	\(102\)	\(133\)
\(\text{KBr}\)	\(138\)	\(196\)

Assume that all other lattice factors are comparable and that the expected melting point increases with the magnitude of the Coulombic attraction between adjacent ions, modeled by

\[ |U|\propto \dfrac{|q_+q_-|}{r_++r_-} \]

Which choice gives the expected melting-point order from greatest to least and best justifies the ranking?