Question 168·Hard·Boundaries
Because the oceanographer cautioned that, by the year 2050, oceanic oxygen levels could drop _____ worrying trend that, if left unchecked, would devastate marine biodiversity, scientists fear the effects could spread through coastal food webs.
Which choice completes the text so that it conforms to the conventions of Standard English?
For boundaries questions, first determine the big structure (independent clause vs. dependent clause). If the text after the blank is a noun phrase that renames the previous idea (an appositive) and the sentence continues afterward, it’s usually set off with commas. Eliminate semicolons unless both sides are independent clauses, and eliminate colons unless the punctuation comes after an independent clause.
Hints
Check whether the sentence starts with a dependent-clause marker
Notice the first word of the sentence. Does it make the opening part an introductory dependent clause?
Decide what kind of words come after the blank
After the blank you see “a worrying trend ...”. Is that a complete sentence, or a noun phrase describing the previous idea?
Use the comma later in the sentence as a clue
Look at the comma after “marine biodiversity.” What does the sentence do immediately after that comma, and what punctuation would normally pair with that earlier?
Step-by-step Explanation
Identify the overall sentence structure
The sentence begins with Because, so everything up to the comma before “scientists fear ...” is an introductory dependent clause. The main clause is “scientists fear the effects could spread through coastal food webs.”
Identify what follows the blank
After the blank comes “a worrying trend ...”, a noun phrase that renames/describes the idea of oxygen levels dropping. That makes it a nonessential appositive, not a new independent clause.
Eliminate punctuation that doesn’t fit this structure
A semicolon separates two independent clauses, but the words after the blank are not an independent clause.
A colon must follow an independent clause; here, the material before the blank is part of a dependent clause introduced by Because, so a colon is not appropriate.
A single em dash would need a matching closing dash later if it starts a parenthetical interruption in the middle of a sentence.
Choose the punctuation that correctly opens the appositive
The comma after “marine biodiversity” closes the appositive and also separates the introductory dependent clause from the main clause (“scientists fear ...”). Therefore, the blank should open the appositive with a comma: “precipitously, a”.