Radiocarbon dating is one of the most reliable methods for determining the age of organic artifacts found at archaeological sites....

Sentence Structure

• Radiocarbon dating is one of the most reliable methods for determining the age of organic artifacts found at archaeological sites.
• The technique measures the decay rate of carbon-14 isotopes in organic material
• A bone fragment that is 5,730 years old will contain exactly half the carbon-14 it had when the organism was alive.

Understanding the Meaning

• The first sentence introduces us to radiocarbon dating:
  • It's described as 'one of the most reliable methods for determining the age of organic artifacts found at archaeological sites.'
  • So it's a way to figure out how old organic things are that archaeologists find.
• Now the second sentence tells us more specifically what this technique does:
  • 'The technique measures the decay rate of carbon-14 isotopes in organic material'
  • This is explaining the actual process - it measures how carbon-14 breaks down in organic material.
• This is where we have the blank. Let's look at the choices:
  • A gives us: semicolon + "for instance,"
  • B gives us: comma + "for instance,"
  • C gives us: nothing (just ends at "material")
  • D gives us: comma only
• To see what works here, let's read what comes after and understand what it's saying!
  • After the blank, we get: 'a bone fragment that is 5,730 years old will contain exactly half the carbon-14 it had when the organism was alive.'
• Let me break down what this is telling us:
  • 'a bone fragment that is 5,730 years old' - a specific example of organic material
  • 'will contain exactly half the carbon-14 it had when the organism was alive'
    • This gives us a concrete illustration of how the decay rate works
    • After 5,730 years, you'll find exactly half the original carbon-14
• So the complete picture is:
  • The first part makes a general statement about measuring decay in organic material
  • The second part gives us a specific example using a bone fragment
• What do we notice about the structure here?
  • Before the blank: 'The technique measures the decay rate of carbon-14 isotopes in organic material'
    • This is a complete thought - it has a subject ("The technique"), a verb ("measures"), and completes the idea.
  • After the blank: 'a bone fragment that is 5,730 years old will contain exactly half the carbon-14...'
    • This is also a complete thought - it has a subject ("a bone fragment"), a verb ("will contain"), and expresses a complete idea.
  • The second complete thought is providing a specific example of the first complete thought.
• When we have two complete thoughts (that could each stand alone as sentences), and the second one is giving us an example of the first, we need:
  • Strong punctuation to properly separate the two complete thoughts (a semicolon)
  • A phrase like "for instance" to signal that an example is coming
• So we need: semicolon + "for instance," (Choice A)

Grammar Concept Applied

Connecting Two Complete Thoughts When One Provides an Example

When you have two complete thoughts (called independent clauses in grammar terms) and the second one provides an example or illustration of the first, you can connect them with a semicolon plus a transitional phrase like "for instance" or "for example":

Pattern:

• Complete thought 1; transitional phrase, complete thought 2.

Example from this question:

• Complete thought 1: "The technique measures the decay rate of carbon-14 isotopes in organic material"
• Semicolon + transitional phrase: ; for instance,
• Complete thought 2: "a bone fragment that is 5,730 years old will contain exactly half the carbon-14 it had when the organism was alive"

Why semicolon + transitional phrase?

• The semicolon provides the strong punctuation needed to separate two complete thoughts
• The transitional phrase "for instance" signals the relationship between the ideas (the second is an example)
• Together, they create a clear, grammatically correct connection

What doesn't work:

• Comma alone → comma splice (too weak to separate complete thoughts)
• No punctuation → run-on sentence
• Comma + transitional phrase → still a comma splice (transitional phrases aren't connecting words like "and" or "but")