A punctuation question with the same words in each choice tests how the sentence's parts connect. Read both sides of the possible mark and test whether they can stand alone: a full sentence needs a subject and a main verb. Then ask how the parts relate. A colon introduces an explanation after a full sentence; a period or semicolon separates full sentences; no mark is needed when the grammar already connects the parts. Test the grammar, not where you'd pause.
Hints
- Hint 1
Cover the words after the possible punctuation mark and read the sentence through the wording the choices share. Can that part stand alone?
- Hint 2
Look for a main verb after the blank. Can involving serve as the main verb without a helper?
- Hint 3
What do the two parts beginning with involving tell you about the claim that the flight requires two metabolic processes?
Step-by-step
Test the sentence and its explanation
Step 1Test the words before the mark
Read through the shared wording up to coordinated, then cover everything after it. Can those words stand alone? The studies say that sustaining a long, nonstop flight requires two processes that are distinct but tightly coordinated. The words that are distinct but tightly coordinated describe the processes rather than leave the claim unfinished. The words before the mark stand alone, but the right punctuation also depends on what follows.
- Step 2
Find the verb after the mark
Do the words after the mark stand alone too? They give us one process involving the release of fatty acids from stored triglycerides and the other involving the oxidation of those fatty acids in muscle cells. The word involving describes each process, but it isn't a main verb, the verb that says what something does or is. An -ing form needs a helper, such as is, to work as the main verb here, and neither description has one. These are two phrases, not a second full sentence.
- Step 3
Attach the explanation
The flight requires two metabolic processes, and the phrases say what those processes are. The versions with a period or a semicolon leave a fragment, a group of words that can't stand alone. With no punctuation, the explanation runs into the completed thought. A colon introduces an explanation after a full sentence, so the version with coordinated: one connects these phrases to the claim they explain. Choice D.