Comma + Because = Wrong
Any answer on the SAT that includes a comma before the word “because” is incorrect. Incidentally, the College Board has been known to ignore this rule in other situations, but it’s always true for Fixing Sentences — the only place it’s directly tested.
In case you’re interested, here’s the rule:
The word “because” is what’s known as a subordinating conjunction, which means any clause that begins with it cannot function as a stand-alone sentence; instead that clause must be joined to an independent clause (a complete, stand-alone sentence).
The need for a comma in a sentence that contains both a dependent and an independent clause is determined by the order of the clauses.
When the independent clause comes first, no comma is usually required. (There are exceptions, but they are not tested.)
Independent clause first: Because it contains buildings from so many different periods, London is a very interesting city to explore. When the dependent clause comes first, a comma is required.
Dependent clause first: London is a very interesting city to explore because it contains buildings from many different periods.
Incorrect: London is a very interesting city to explore, because it contains buildings from so many different periods.