Consider the structure of complex sentences and the use of relative clauses in English. Analyze the following statements:
I.Defining relative clauses provide essential information about the noun they modify and are never separated from the rest of the sentence by commas.
II.Non-defining relative clauses provide extra information and must be enclosed in commas, but the relative pronoun "that" cannot be used in these structures.
III.In informal English, it is grammatically incorrect to end a sentence with a preposition, even in relative clauses where the pronoun is omitted.
Which of the statements above is/are CORRECT?