Which of the following is not a correct statement about a derivation?

A derivation is a repeated application of rules, starting with the start symbol and ending with a sentence.

Every string of symbols in a derivation is a sentential form.

A sentence is a sentential form that has only terminal symbol.

A leftmost derivation is one in which the leftmost nonterminal in each sentential form is the one that is expanded.

A derivation is either leftmost or rightmost derivation