Question 107·Medium·Boundaries
During sudden volcanic eruptions, airborne ash can travel thousands of miles; particles that remain suspended in the atmosphere scatter sunlight, causing sunsets to appear unusually vivid while _____ gradually settling back to Earth.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation questions like this, first identify what comes immediately before and after the underlined portion, and decide whether you have a complete sentence (independent clause) on each side. Quickly rule out colons and semicolons if there is no full sentence before them. Then decide whether the words you’re separating are equal modifiers (like pairs of adjectives or adverbs); if they are, a comma often correctly separates them, while no punctuation can make the phrase awkward or unclear. Always read the full sentence with each choice to check for both grammatical correctness and natural, clear wording.
Hints
Focus on what comes before the blank
Look carefully at the words right before and after the blank: "while ___ gradually settling back to Earth." What kind of word will go in the blank, and what is it describing?
Think about colon and semicolon rules
Recall when you are allowed to use a colon or a semicolon. Do you have a complete sentence right before the punctuation mark in this spot?
Compare punctuation between modifiers
You will end up with two describing words in a row before "settling." Which punctuation mark is normally used (if any) between two equal describing words that both modify the same action?
Step-by-step Explanation
Understand the structure of the sentence
Read the key part of the sentence:
"...causing sunsets to appear unusually vivid while _____ gradually settling back to Earth."
The blank comes before "gradually settling back to Earth." The word in the blank describes how the particles are settling. So we will end up with two describing words (adverbs) in a row before "settling": one in the blank and "gradually." We need punctuation that correctly links those two modifiers.
Eliminate colon and semicolon options
Look at what comes before the punctuation mark in the answer choices: the word "slowly" inside the phrase "while slowly ..."
- A colon (:) must come after a complete sentence (an independent clause) and then introduce a list, explanation, or example. "While slowly" is not a complete sentence, so it cannot be followed by a colon.
- A semicolon (;) must go between two complete sentences. Here, before the punctuation we only have a single word, "slowly," not a full sentence.
So any choice that uses a colon or semicolon cannot follow the rules of Standard English in this spot.
Choose between comma and no punctuation
Now compare having no punctuation versus a comma between the two adverbs:
- With no punctuation, we get "while slowly gradually settling back to Earth." This sounds awkward and makes it unclear how the modifiers relate; it looks like "gradually" might be modifying "slowly" instead of both modifying "settling."
- With a comma, we get two coordinate (equal) adverbs: "slowly, gradually settling back to Earth," which is like saying "slowly and gradually settling..." This clearly and smoothly shows two separate ways the particles are settling.
Therefore, the choice that uses a comma after "slowly" is the one that correctly follows Standard English conventions.