A punctuation question with different comma placements tests which words need to stay together. Read across the blank and name the parts each comma would separate. A comma before and can join two full sentences, but it doesn't separate two noun groups joined by and. A that clause, a group starting with that and having its own verb, stays attached to the word it explains, with no comma before or after that. Check how the words connect, not where you'd pause while reading.
Hints
- Hint 1
Start with because of and ask what reasons the sentence gives. Follow those reasons through the blank.
- Hint 2
Check the link into the last part of the sentence. Which words lead into “their arrival times can shift quickly,” and what job do they do?
Step-by-step
Name the parts at each comma
Step 1Find what completes the reason
Start with is difficult because of the dozens of and read across the blank. Since every choice has the same words, we're deciding where commas can go without splitting words that belong together. What completes because of? The words answering because of what? are its object. They give the reasons for the difficulty, so the reasons continue across the blank.
- Step 2
See what and joins
The word and joins the routes the drivers can take with the fact that their arrival times can change. Are these full sentences or parts of the reason introduced by because of? Although both groups contain verbs, they're built around the nouns routes and fact, so they're noun groups rather than full sentences. A comma before and joins two full sentences, so it doesn't belong between these groups. A comma after and would cut the joining word off from the second group.
- Step 3
Connect the fact to what explains it
Read into their arrival times can shift quickly: how are those words linked to fact? The word that starts a that clause, a group with its own subject and verb, which tells us what the fact is. No comma belongs after that, so the version with no commas keeps this clause attached and joins the two noun groups without a break. Choice D.