By analyzing the level of radioactive decay within a fossil specimen, scientists can establish the age of that fossil with...

Sentence Structure

First sentence:

• By analyzing the level of radioactive decay within a fossil specimen,
• scientists can establish the age of that fossil with a high degree of precision.

Second sentence:

• When radioactive elements aren't present,
• scientists turn to stratigraphy[?]the analysis of Earth's sediment layers (strata) - to estimate how old a fossil is based on the age of the strata in which the fossil is found.

The blank: stratigraphy[?]the

• Common: "stratigraphy" and "the"
• What varies: [?] = the punctuation between them

Understanding the Meaning

Let's start with the first sentence:

• "By analyzing the level of radioactive decay within a fossil specimen, scientists can establish the age of that fossil with a high degree of precision."
• This tells us one method scientists use: measuring radioactive decay to date fossils very accurately.

Now the second sentence:

• "When radioactive elements aren't present,"
  • This sets up an alternative scenario - what if radioactive elements aren't available to analyze?
• "scientists turn to..."
  • They use a different method

This is where we have the blank: "stratigraphy[?]the analysis of Earth's sediment layers (strata)"

Let's look at the choices:

• A: em dash (—)
• B: opening parenthesis (
• C: colon (:)
• D: comma (,)

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

The full phrase reads: "scientists turn to stratigraphy[?]the analysis of Earth's sediment layers (strata)—to estimate how old a fossil is based on the age of the strata in which the fossil is found."

Now let's understand what this is telling us:

• "Stratigraphy"
  • This is a technical/scientific term
• "The analysis of Earth's sediment layers (strata)"
  • This is explaining what stratigraphy means - it's defining the term for us
  • "(strata)" is just another word for sediment layers, given in parentheses
• "To estimate how old a fossil is based on the age of the strata in which the fossil is found"
  • This explains the purpose - scientists use stratigraphy to estimate fossil age by looking at the age of the rock layers where the fossil was found

So the complete picture is:

• The sentence is saying "scientists turn to stratigraphy to estimate how old a fossil is"
• But mid-sentence, it inserts an explanation of what stratigraphy is: "the analysis of Earth's sediment layers (strata)"

What do we notice about the structure here?

• Notice there's already an em dash before "to estimate" in the original sentence
• This means the explanation "the analysis of Earth's sediment layers (strata)" sits between two points in the sentence
• If we removed that explanation, we'd have: "scientists turn to stratigraphy—to estimate how old a fossil is"
  • This works! The main sentence is complete without the explanation.
• So the explanation is an interruptive phrase - inserted information that explains the technical term
• This kind of interruption needs to be bracketed - opened and closed - with matching punctuation
• The closing punctuation is already there (the em dash before "to estimate")
• So we need an opening em dash after "stratigraphy" to match it

The structure is:
"scientists turn to stratigraphy—the analysis of Earth's sediment layers (strata)—to estimate how old a fossil is"

The correct answer is Choice A (em dash). We need a pair of em dashes to bracket the explanatory phrase that's been inserted into the middle of the sentence.

GRAMMAR CONCEPT APPLIED

Using Paired Em Dashes to Set Off Interruptive Explanations

When you insert an explanation, definition, or clarifying phrase into the middle of a sentence, you can bracket it with a pair of em dashes. This punctuation choice signals "I'm adding extra information about what I just mentioned, then I'll return to my main point."

The key test: Remove what's between the em dashes. The main sentence should still be grammatically complete.

Here's how it works:

Example 1:

• With explanation: "The scientist studied tardigrades—microscopic animals that can survive extreme conditions—for over a decade."
• Without explanation: "The scientist studied tardigrades for over a decade." ✓ Complete!
• The phrase "microscopic animals that can survive extreme conditions" is the interruptive explanation

Example 2:

• With explanation: "The committee chose stratigraphy—the analysis of rock layers—as their primary dating method."
• Without explanation: "The committee chose stratigraphy as their primary dating method." ✓ Complete!
• The phrase "the analysis of rock layers" defines what stratigraphy is

In our question:

• With explanation: "scientists turn to stratigraphy—the analysis of Earth's sediment layers (strata)—to estimate how old a fossil is"
• Without explanation: "scientists turn to stratigraphy to estimate how old a fossil is" ✓ Complete!
• The phrase "the analysis of Earth's sediment layers (strata)" explains what stratigraphy means

Why em dashes specifically?
Em dashes are particularly good for:

• Drawing attention to the inserted information
• Setting off longer or more substantial explanations
• Creating a clear interruption that's easy to spot visually

Important: The punctuation must be paired and matching. If you use an em dash to open the interruption, you must use an em dash to close it. You cannot mix punctuation marks (like comma + dash).