Question 243·Hard·Boundaries
Bioluminescent organisms—fireflies, some deep-sea fish, and certain species of fungi—produce light through chemical reactions, a process scientists call chemiluminescence; they use this _____, to attract mates or deter predators.
Which choice completes the text so that it conforms to the conventions of Standard English?
First examine the punctuation already present around the blank. If the missing words form a nonessential interruption, ensure they’re enclosed by a consistent pair of punctuation marks. Eliminate options that create double commas or mix commas with dashes, and choose the one that makes the boundaries clean and consistent.
Hints
Look after the blank
Notice that there is already a comma immediately after the blank. What punctuation do you need before the interruption to make a matching pair?
Recall how nonessential phrases are set off
Nonessential interruptions should be enclosed by the same punctuation on both sides: commas, dashes, or parentheses.
Avoid punctuation collisions
Eliminate any option that would produce a double comma (,,) or put a comma right next to a dash.
Step-by-step Explanation
Identify the role of the missing words
The blank introduces specific uses of the light (attracting mates, deterring predators). This is an interrupting phrase that provides an example, so it should be set off as a nonessential element.
Check the surrounding punctuation
In the sentence, a comma already follows the blank: they use this _____, to attract.... That means the answer should complete a matched pair of commas around the interruption by supplying the comma before it.
Apply the matching-boundary rule
Nonessential elements must be framed by a consistent pair of punctuation marks (commas, dashes, or parentheses). Avoid creating a double comma (,,) or mixing punctuation marks (like a dash touching a comma).
Test the choices
- light, for example → they use this light, for example, to attract... (balanced commas; correct)
- light, for example, → they use this light, for example,, to attract... (double comma)
- light—for example— → they use this light—for example—, to attract... (dash touching a comma; mixed punctuation)
- light—for example, → they use this light—for example,, to attract... (dash plus double comma; incorrect)
Therefore, the correct answer is light, for example.