Ecologists studying the rapid decline of monarch butterfly populations across North America have determined that the contributing factors are _____...

Sentence Structure

• Ecologists studying the rapid decline of monarch butterfly populations across North America
  • have determined
    • that the contributing factors are threefold [?]
      • habitat destruction along migration routes,
      • increased pesticide exposure in agricultural areas,
      • and climate change disrupting seasonal patterns.

Understanding the Meaning

Let's read from the beginning to understand what this sentence is telling us:

'Ecologists studying the rapid decline of monarch butterfly populations across North America'

• These are scientists who study ecosystems
• They're specifically studying why monarch butterflies are declining rapidly
• This is happening across North America

'have determined that the contributing factors are threefold'

• The ecologists have figured out what's causing this decline
• There are three contributing factors (that's what "threefold" means)

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

• We're deciding what punctuation should come after "threefold"
• Our options are: no punctuation, a colon, a period, or a semicolon

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

'habitat destruction along migration routes, increased pesticide exposure in agricultural areas, and climate change disrupting seasonal patterns'

This is giving us a list of three things:

• habitat destruction along migration routes
• increased pesticide exposure in agricultural areas
• climate change disrupting seasonal patterns

So the complete picture is:

• The sentence says there are three factors (threefold)
• Then it lists exactly what those three factors are

What do we notice about the structure here?

The first part - "the contributing factors are threefold" - is a complete thought. It has a subject (factors) and verb (are) and could stand alone as a sentence.

The second part - the list of three factors - is NOT a complete thought:

• These are just noun phrases (things/concepts)
• There's no verb making them complete sentences
• They're just naming what the three factors are

The relationship between these parts is key:

• "Threefold" announces that three things are coming
• The list provides the specific three things
• The list explains and elaborates on what "threefold" means

When we have a complete thought followed by a list that explains or gives details about something just mentioned, we need a colon. The colon signals "here's what I just referenced" or "here come the specifics."

So we need a colon. The correct answer is B.

GRAMMAR CONCEPT APPLIED

Using Colons to Introduce Lists and Explanations

A colon is used after a complete statement to introduce a list, explanation, or elaboration that provides specific details about something just mentioned. The colon signals "here's what I mean" or "here are the specifics."

The pattern looks like this:

Structure:

• Before the colon: Complete thought (independent clause with subject + verb)
• After the colon: List, explanation, or elaboration (doesn't need to be a complete sentence)

Example 1:

• The recipe requires three ingredients: flour, sugar, and eggs.
• "The recipe requires three ingredients" = complete thought
• "flour, sugar, and eggs" = list providing the specifics (not a complete sentence)

Example 2:

• She had one goal: to win the championship.
• "She had one goal" = complete thought
• "to win the championship" = explanation of the goal (not a complete sentence)

In this question:

• "The contributing factors are threefold" = complete thought announcing three things
• The list of three factors = the specific details explaining what "threefold" means
• The colon connects them properly: announces → provides specifics

Key point: The part after the colon doesn't need to be a complete sentence. It just needs to explain, list, or elaborate on what came before the colon.