Who is used to refer to persons; which refers to things. SAT designed in a way to trick you. You will generally only find “that” used to introduce a restrictive (or defining) clause (one that identifies the specific thing) or used as a conjunction.

       E.g. The book that you need is on the second shelf.

                I’m lucky that I found it in time.

The relative pronoun “which” is used on the SAT for non-restrictive, non-defining clauses, set off by commas.

       E.g. The book, which is now out of print, is on the second shelf.

*If the modifying phrase is set off by one or more commas, you probably need to use “which” instead of that.