It is misleading and oversimplistic to say that "which" must always be preceded by a comma. The rule is that a nonrestrictive clause should be set off with commas, but a nonrestrictive clause does not always begin with "which," and the presence of "which" does not always signal a nonrestrictive clause.
First of all, if the relative pronoun is the object of a preposition, whether the clause is restrictive or nonrestrictive, you cannot use "that"; you must use "which." Here are a couple of correct sentences from the Official Guide that use "which" after a preposition in a restrictive clause:
El Nino, the periodic abnormal warming of the sea surface off Peru, is a phenomenon in which changes in the ocean and atmosphere combine to allow the warm water that has accumulated in the western Pacific to flow back to the east.
As U.S. nuclear attack submarines prowl their familiar haunts deep within the oceans of the world these days, they increasingly are engaged in missions far different from the tasks for which they were built and for which their crews were trained over the last forty years.
Second, in contemporary English usage, "which" is very often used where traditionalists would prefer "that." According to The Little, Brown Handbook:
"Which" can introduce both restrictive and nonrestrictive clauses, but many writers reserve "which" only for nonrestrictive clauses.
Still, what concerns us here is what GMAT writers think, and if you look at the explanation to OG11 SC7 you will see that choice A is wrong in part because "Which is incorrectly used for a clause that defines," and that choice C is wrong in part because "Which incorrectly introduces a restrictive clause." It is worth noting, however, that both of these wrong choices have other problems; misuse of "which" is not the only reason to eliminate them.
The fact is that a study of edited prose has shown that about 75 percent of the instances of "which" introduce restrictive clauses and only about 25 percent introduce nonrestrictive clauses.
OP- Bob@urch forum
