Question 69·Hard·Boundaries
An extended drought can reduce river flow rates, limiting hydroelectric power output and, by _____ energy supply to nearby communities.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation and boundary questions, first identify whether any words in the middle of the sentence are extra or interrupting the main idea (parenthetical phrases like "however," "for example," or "by extension"). Check what punctuation already appears before that phrase, and remember that interrupters must be set off with matching punctuation on both sides (commas with commas, dashes with dashes). Quickly plug in each option and read the sentence in your head; eliminate any choice that creates mismatched, doubled, or awkward punctuation, and choose the one that keeps the sentence smooth and grammatical.
Hints
Spot the extra phrase
Look at the words around the blank. Is there a short phrase that interrupts the flow between "and" and "energy supply" and adds extra explanation?
Check what punctuation is already there
Notice the punctuation mark immediately before "by." Whatever you put in the blank should work together with that mark to surround the interrupter phrase.
Match the punctuation pair
For interrupter phrases like this, should you mix different punctuation marks on each side, or should they match? Eliminate any choices that create mismatched or doubled-up punctuation.
Step-by-step Explanation
Identify the interrupter phrase
Read the sentence around the blank:
"...limiting hydroelectric power output and, by _____ energy supply to nearby communities."
The words "by extension" interrupt the main idea (they are extra explanatory information between "and" and "energy supply"). That makes "by extension" a parenthetical or interrupter phrase that needs to be set off with punctuation on both sides.
Recall the rule for setting off interrupters
In Standard English, when you set off an interrupter like "by extension," you must:
- Use punctuation on both sides of the interrupter.
- Use the same kind of punctuation mark on both sides (both commas, or both dashes).
In the sentence, we already have a comma before the interrupter (right after "and"). So after "extension" we need matching punctuation that completes the pair.
Test each answer choice for correct, matching punctuation
Plug in each option and check whether the punctuation cleanly and correctly sets off the interrupter:
- A) "extension—" → mixes a comma before the interrupter and a dash after; the marks do not match.
- B) "extension,—" → creates a comma plus dash together after the phrase; this is not standard and creates a punctuation pile-up.
- C) "extension—," → creates a dash immediately followed by a comma; this double punctuation is incorrect.
- D) "extension," → uses commas on both sides of the interrupter, which correctly sets off "by extension."
Therefore, the correct answer is "extension,".