Researchers studying magnetosensation have determined why some soil-dwelling roundworms in the Southern Hemisphere move in the opposite direction of E...

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

• Researchers studying magnetosensation
  • have determined why some soil-dwelling roundworms in the Southern Hemisphere
    • move in the opposite direction of Earth's magnetic field
      • when searching for food [?]
• in the Northern Hemisphere,
  • the magnetic field points down,
    • into the ground,
• but in the Southern Hemisphere,
  • it points up,
    • toward the surface and away from worms' food sources.

Understanding the Meaning

Let's start reading and understanding:

The sentence begins by telling us about some researchers:

• 'Researchers studying magnetosensation'
  • These are scientists who study how creatures sense magnetic fields
• 'have determined why some soil-dwelling roundworms in the Southern Hemisphere move in the opposite direction of Earth's magnetic field when searching for food'
  • They figured out the REASON why certain worms move against the magnetic field
  • This is interesting - they determined "why" something happens
  • This creates an expectation: we want to know what that reason is!

This is where we have the blank.

Let's look at the choices:

• We're deciding between a colon, a comma, the word "while", or no punctuation
• This tells us we need to figure out what kind of connection is needed here

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

'in the Northern Hemisphere, the magnetic field points down, into the ground, but in the Southern Hemisphere, it points up, toward the surface and away from worms' food sources.'

Now let's understand what this is telling us:

• 'in the Northern Hemisphere, the magnetic field points down, into the ground'
  • In the Northern Hemisphere, the magnetic field goes downward - into the soil
  • For worms that live in soil, this is good - they can follow it to their food
• 'but in the Southern Hemisphere, it points up, toward the surface and away from worms' food sources'
  • In the Southern Hemisphere, it's the opposite - the field points upward
  • This points AWAY from where the worms need to go for food
  • So the worms have to move AGAINST the field to get to their food

So the complete picture is:

• The first part says researchers figured out WHY the worms behave a certain way
• The second part PROVIDES THAT EXPLANATION - it tells us the actual reason

What do we notice about the structure here?

• Both parts are complete thoughts
  • The first part: "Researchers have determined why..." - complete sentence
  • The second part: "in the Northern Hemisphere, the magnetic field points down...but in the Southern Hemisphere, it points up..." - also complete
• The second part is explaining something promised in the first part
  • The first part mentions they determined "why"
  • The second part delivers the explanation of that "why"

When we have a complete thought followed by another complete thought that explains or elaborates on it, we need a colon.

So we need food:

The correct answer is A.

GRAMMAR CONCEPT APPLIED

Using a Colon to Introduce an Explanation

When you have a complete thought that mentions or promises something, and then you want to provide the explanation or details about it, you use a colon. The part before the colon must be a complete thought (called an independent clause in grammar terms), and what follows elaborates on or explains it.

The pattern:

• [Complete thought that mentions/promises something] : [Explanation or elaboration]

Examples:

Example 1:

• "The scientist discovered why the experiment failed: the temperature had been set too high."
• Before colon: Complete thought mentioning a "why"
• After colon: The explanation of that "why"

Example 2:

• "The results were clear: students who practiced daily improved by 40%."
• Before colon: Complete thought mentioning "results"
• After colon: The specific results

In our question:

• Before colon: "Researchers determined why some roundworms move in the opposite direction... when searching for food"
• Complete thought promising an explanation
• After colon: "in the Northern Hemisphere, the magnetic field points down... but in the Southern Hemisphere, it points up..."
• The explanation of why the worms behave that way

The colon signals: "Here comes the explanation I just mentioned!"