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) Ar > Kr because of the lower atomic mass and stronger dispersion forces of Ar.
b) Al > H2O due stronger dispersion forces in Al than in H2O.
c) CO2 > SiO2 because SiO2 is a covalent-network solid, and CO2 is molecular.
d) MgF2 > LiF due to higher charge on Mg2+ than Li+.
e) BeO > MgS due ionic bonding in BeO not present in MgS.