The melting point of $RbBr$ is $682\,^{\circ}C$ while that of $NaF$ is $988\,^{\circ}C$. The principal reason for this fact is

• A
  the molar mass of $NaF$ is smaller than that of $RbBr$
• B
  the bond in $RbBr$ has more covalent character than the bond in $NaF$
• C
  the difference in electronegativity between $Rb$ and $Br$ is smaller than the difference between $Na$ and $F$
• D
  the internuclear distance,$r_c + r_a$ is greater for $RbBr$ than for $NaF$