Which of the following is not a rule that must be followed when naming identifiers $($variables$)?$

After the first character, you may use the letters a$-$z$,$ A$-$Z$,$ an underscore, a dollar sign, or the digits $0-9.$

Uppercase and lowercase characters are distinct.

Identifiers can contain spaces.

The first character must be one of the letters $a-Z,A-Z,$ an underscore, or a dollar sign.