The books which the professor assigned were very expensive.

Explanation

- "which the professor assigned" is an adjective clause (also known as a relative clause) because: - It begins with the pronoun "which" - It has a subject (the professor) and a verb (assigned) - It functions as an adjective, modifying the noun "books" and providing additional information about which books are being referred to - It cannot stand alone as a complete sentenceThe adjective clause "which the professor assigned" describes the books and indicates which specific books were assigned by the professor.Note: Adjective clauses typically begin with pronouns like "which", "that", or "who", and modify nouns or pronouns. In this case, the adjective clause "which the professor assigned" modifies the noun "books".