A group of ecologists led by Axel Mithöfer at the Max Planck Institute for Chemical Ecology in Germany examined the...

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

• A group of ecologists
• led by Axel Mithöfer at the Max Planck Institute
  • for Chemical Ecology in Germany
• examined the defensive responses
  • of two varieties of the sweet potato plant (?)
    • TN57,
      • which is known for its insect resistance,
    • and TN66,
      • which is much more susceptible to pests.

Understanding the Meaning

Let's start from the beginning:

A group of ecologists did something - they:

• 'examined the defensive responses of two varieties of the sweet potato plant'
  • They studied how two different types of sweet potato plants defend themselves

This is where we have the blank - right after 'plant.'

Let's look at the choices:

• We're deciding what punctuation (if any) should come after 'plant'
  • A period, semicolon, nothing, or colon

To see what works here, let's read what comes next and understand what it's saying!

After the blank, we get:

• 'TN57, which is known for its insect resistance, and TN66, which is much more susceptible to pests.'

Now let's understand what this is telling us:

• 'TN57, which is known for its insect resistance'
  • This is naming the first variety
  • It adds the detail that TN57 is resistant to insects

• 'and TN66, which is much more susceptible to pests'
  • This is naming the second variety
  • It adds the detail that TN66 is NOT resistant - pests can attack it easily

So the complete picture is:

• The sentence mentions 'two varieties of the sweet potato plant'
• Then it names exactly what those two varieties ARE: TN57 and TN66

What do we notice about the structure here?

• The sentence makes a general statement: 'two varieties of the sweet potato plant'
  • Then it gives us the SPECIFIC list of what those varieties are

• What comes after the blank is a list that specifies/names the two varieties
  • It's not a complete sentence on its own
  • It's providing the specific examples of what was just mentioned

This is a classic list-introduction situation. When you mention something general and then provide the specific list or names, you use a colon to introduce that list.

The correct answer is D (plant:)

Grammar Concept Applied

Using a Colon to Introduce a List or Specification

When your sentence mentions something in a general way and then provides the specific examples or list of what that thing is, use a colon to introduce the specification (this punctuation mark is called a colon in grammar terms):

Pattern:

• General statement: The team studied three methods
• Colon introduces list: The team studied three methods: filtration, distillation, and evaporation
  • Before colon = complete thought mentioning 'three methods'
  • After colon = specific list naming what those methods are

Another example:

• General statement: The museum features art from two periods
• Colon introduces specification: The museum features art from two periods: the Renaissance and the Baroque
  • Before colon = complete thought mentioning 'two periods'
  • After colon = names exactly which two periods

In this question:

• Before colon: 'examined the defensive responses of two varieties of the sweet potato plant'
  • Complete thought mentioning 'two varieties'
• After colon: 'TN57, which is known for its insect resistance, and TN66, which is much more susceptible to pests'
  • Specifies which two varieties

The colon signals: 'Here comes the specific information about what I just mentioned.'