Arrange the following ionic compounds in order of increasing melting points.
NaF, MgF2, AlF3, NaBr, NaI
a. NaF < NaBr < NaI < MgF2 < AlF3
b. NaBr < NaI < NaF < AlF3 < MgF2
c. MgF2 < AlF3 < NaF < NaI < NaBr
d. AlF3 < MgF2 < NaF < NaBr < NaI
e. NaI < NaBr < NaF < MgF2 < AlF3
Answer: E