Which of the following statements is incorrect?

Group of answer choices

Simple languages can be expressed by a formula involving languages containing a single string of length $1$ and the operations of union, concatenation and Kleene star are regular languages.

Context$-$free languages must be regular languages.

Regular languages must be context$-$free languages.

A grammar is $$context free$$ in the sense that its rules do not say anything about the context in which each piece of syntax may appear.