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) SeO2 > SiO2 due to the higher molar mass of SeO2. b) NH3 > PH3 due ionic bonding in NH3 not present in PH3. c) CO2 > SiO2 because SiO2 is a covalent-network solid, and CO2 is molecular. d) SiF4 > Si due to the higher molar mass of SiF4. e) MgF2 > NaF due to the higher charge on Mg2+ than on Na+.