Boundaries on the Digital SAT

Run the independent clause test on each side. "Kelp is one of the fastest-growing organisms on the planet" stands alone, and so does "a single blade can lengthen by more than a foot in one day." Two full sentences means only the period works, since a comma alone would be a splice.

Both sides of the blank are independent clauses: "Kelp is one of the fastest-growing organisms on the planet" and "a single blade can lengthen by more than a foot in one day..." each have their own subject and verb and can stand alone. Two independent clauses must be separated by a period, a semicolon, or a comma plus a coordinating conjunction. Choice A uses a period, which does the job cleanly. Choice B creates a comma splice: a comma alone cannot join two complete sentences. Choice C fuses the two sentences with no punctuation at all, producing a run-on. Choice D adds "and" but wraps it in a stray comma ("and, a"), which incorrectly separates the conjunction from the clause it introduces.

Same independent clause test, except "Although" is the tell. It turns everything before the blank into a clause that cannot stand on its own, so a semicolon or period is out. A dependent opener attaches to the main clause with a comma, which is why "residents, riders" wins.

The word "Although" makes everything up to the blank a dependent clause: "Although the city's original charter promised that the ride would stay free for residents" cannot stand alone as a sentence. A dependent clause that opens a sentence must be attached to the main clause with a comma, which is exactly what choice A does. Choice B is wrong because a semicolon can only sit between two independent clauses, and the first part is dependent. Choice C punctuates the dependent clause as its own sentence, creating a fragment. Choice D omits the required comma after a long introductory clause, running the two parts together.

This is "Scan the passage for a pattern" at work. The list already separates items with semicolons later ("four generations; and Sam Okafor") because each baker comes with its own internal comma. Match that pattern and the blank takes a semicolon too.

This is a list in which each item already contains commas ("Marisol Vega, who runs a small panadería in San Antonio"). When list items have internal commas, the items themselves must be separated by semicolons so readers can tell where one item ends and the next begins. The passage confirms the pattern later with "Milwaukee for four generations; and Sam Okafor." Choice A matches that pattern. Choice B uses a comma, which makes the boundary between the first and second bakers ambiguous and breaks the established semicolon pattern. Choice C is wrong because a colon cannot separate items within a list; the colon's job here was already done after "three bakers." Choice D ends the sentence early, leaving everything after it as a fragment with no main clause.