#7 · Let the opener decide
The question. Which choice completes the text so that it conforms to the conventions of Standard English?
The octopus—an animal with three hearts and blue ______ can change its skin color in a moment.
- blood,
- blood—
- blood;
- blood
The quick move, and where it breaks. On a punctuation question, the quick move is to read the words on each side of the blank. Here that gives "blue blood, can change color," which sounds fine, so A looks right.
But this sentence has extra information dropped into the middle: an animal with three hearts and blue blood. Take it out and the sentence still works on its own: "The octopus can change its skin color in a moment." Extra information in the middle of a sentence gets a matching pair of marks, one to open it and one to close it. Here, a dash opened it, right after "octopus," so a dash has to close it. The answer is B.
The comma in A is the trap: it reads naturally next to the blank, but it would close with a comma what a dash opened. The semicolon in C would need a complete sentence on each side, and D leaves the dash with no partner.
The same rule, three ways
Change the opener, and the closing punctuation changes too:
- The octopus, an animal with three hearts and blue blood, can change its skin color in a moment.
- The octopus—an animal with three hearts and blue blood—can change its skin color in a moment.
- The octopus (an animal with three hearts and blue blood) can change its skin color in a moment.
All three are correct. What is never correct is a mismatch: a dash on one side and a comma on the other. So don't decide at the blank. Find the mark that opened the extra information and let it decide.
Two cases the video leaves out
- An aside at the end of a sentence has no closer. "The octopus has three hearts, a fact that surprises most people." The period does the closing. The rule is about extra information in the middle.
- A single dash can work like a colon. "The octopus has one hard part—its beak." One dash, no partner, correct. A student who learns "dashes come in pairs" as an absolute will second-guess this.
The takeaway: let the opener decide. When extra information sits in the middle of a sentence, the mark that closes it has to match the mark that opened it.