Which of the following statements is correct about the periodic table?

A.
Elements in the same group have the same number of electron shells
B.
The non-metallic properties of the elements tend to decrease across each period
C.
Elements in the same period have the same number of valence electrons
D.
The valence electrons of the elements increase progressively across the period

There is an explanation video available below.

What is the mean of the data t, 2t-1, t-2, 2t-1, 4t and 2t+2?