That vs. Which

"Expendable--Which

If you leave off the clause that says which is bad news, it doesn't change the meaning of the rest of the sentence. A quick and dirty tip (with apologies to Wiccans and Hermione Granger) is to remember that you can throw out the “whiches” and no harm will be done. You use which in nonrestrictive clauses, and if you eliminate a nonrestrictive clause, the meaning of the remaining part of the sentence will be the same as it was before.... On the other hand, if it would change the meaning to throw out the clause, you need a that."
Mignon Fogarty is the host of Grammar Girl.