Choose the answer which presents the chemical formulas for two substances in order of decreasing melting point (highest melting point first) and gives a valid reason for this order.
a) MgF2 - NaF due to the higher molar mass of MgF2.
b) H2O - Al due to hydrogen-bonding in H2O not present in Al.
c) CaO - ScN due to ionic forces in CaO not present in ScN.
d) SiF4 - Si due to the higher molar mass of SiF4.
e) BeO - MgS due to smaller ions present in BeO leading to larger lattice energy.