All of the elements in a family or group have similar chemical properties. Which of the following statements best explains why this is true?

AThe elements in a group have the same number of protons in their nuclei.

BThe elements in a group have similar atomic mass numbers.

CThe elements in a group have the same number of valence electrons.

DThe elements in a group have similar atomic numbers.

I don't remember... Don't have my notes either.