Which of the following statement about the periodic table is correct.

The atomic masses of three elements $\mathrm{X}, \mathrm{Y}$ and $\mathrm{Z}$ having similar chemical properties are $7, 23$ and $39$ respectively.

a) Calculate the average atomic mass of elements $\mathrm{X}$ and $\mathrm{Z}$.

b) How does the average atomic mass of elements $\mathrm{X}$ and $\mathrm{Z}$ compare with the atomic mass element of $\mathrm{Y}$?

c) Which law of classification of elements is illustrated by this example?

d) What could the elements $\mathrm{X}$, $\mathrm{Y}$ and $\mathrm{Z}$ be?

e) Give another example of set of elements which can be classified according to this law.