A study led by scientist Rebecca Kirby at the University of Wisconsin–Madison found that black bears that eat human food...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

Sentence 1:

• A study led by scientist Rebecca Kirby at the University of Wisconsin-Madison
  • found that
    • black bears that eat human food before hibernation
      • have increased levels of a rare carbon isotope,
        • carbon-13 (?) ( C) (?)
        • due to the higher C levels in corn and cane sugar.

Sentence 2:

• Bears with these elevated levels
  • were also found to have much shorter hibernation periods on average.

Understanding the Meaning

Let's start reading from the beginning:

A study found that:

• black bears that eat human food before hibernation
  • have increased levels of a rare carbon isotope...

This is where we have the blank. Let's look at the choices:

• All choices contain "carbon-13 ( C)"
• They differ in comma placement -
  • whether there's a comma between carbon-13 and ( C)
  • whether there's a comma after ( C)

To see what works here, let's read the rest of the sentence and understand what it's saying!

The sentence continues:

• "due to the higher C levels in corn and cane sugar"

Now let's really understand the structure here:

• "carbon-13 ( C)"
  • This is identifying WHICH carbon isotope we're talking about
  • "Carbon-13" is the name
  • "( C)" is the scientific notation for the same thing
  • These work together as ONE naming unit
    • Like "New York City" or "Dr. Jane Smith"
    • You don't split up a name or identifier with commas

• "due to the higher C levels in corn and cane sugar"
  • This is a separate piece of information
  • It's explaining WHY the bears have increased levels
  • It's additional explanation, not part of the identification

What do we notice about what's needed?

• "Carbon-13" and "( C)" should stay together as one unit
  • No comma between them

• But the "due to" phrase is separate explanatory information
  • It needs to be set off from the identification
  • So we need a comma AFTER "( C)" to separate these two functions

Let me test this: If I remove "due to the higher C levels in corn and cane sugar," does the sentence still work?

• "Bears have increased levels of a rare carbon isotope, carbon-13 ( C)."
• Yes! This is complete.
• This confirms the "due to" phrase is additional info that should be separated.

So we need: carbon-13 ( C), - keeping the naming unit together, but separating it from the explanatory phrase that follows.

The correct answer is Choice D.

GRAMMAR CONCEPT APPLIED

Using Commas with Compound Identifiers and Explanatory Phrases

When you have a compound identifier - a name or term that includes multiple parts working together as one unit (sometimes called a compound noun or technical term) - keep all parts together without internal commas. However, when you add explanatory or descriptive information after that identifier, separate it with a comma.

Pattern:

Compound Identifier (kept together):

• "carbon-13 ( C)" - name + scientific notation = one unit
• "Dr. Jane Smith" - title + name = one unit
• "New York City" - parts of one place name = one unit

Adding Explanatory Information (separated with comma):

• Original: "The sample contained carbon-13 ( C)"
• With explanation: "The sample contained carbon-13 ( C), a rare isotope found in processed foods"
• "carbon-13 ( C)" = the identifier (no internal comma)
• comma after ( C) = separates the explanation that follows

In this question:

• "a rare carbon isotope, carbon-13 ( C), due to..."
• First comma: introduces which isotope (already there)
• "carbon-13 ( C)": the complete identifier stays together
• Second comma: separates the "due to" explanation from the identification

Why this matters:
Keeping compound identifiers together while properly separating explanatory phrases helps readers distinguish between what something IS (its identification) versus additional information about it (explanation or description).