A. I wanted to double check this before I answered you. My answer is a paraphrase of The Gregg Reference Manual. "Which" is used to introduce nonessential clauses. "That" ordinarily introduces essential clauses.
My brother-in-law's experience, which was limited at best, did not reassure me. ("Which" introduces a nonessential clause.)
The book that I lent you used to belong to my grandfather. ("That" introduces an essential clause.)
However, Gregg goes on to state that many writers now use either "which" or "that" to introduce an essential clause, especially if