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

Question $5$ options:

A$)$

Uppercase and lowercase characters are distinct.

B$)$

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

C$)$

Identifiers can contain spaces.

D$)$

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