Many prey animals instinctively freeze when threatened by _____ do so reduces their visibility to predators that rely primarily on...

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

• Many prey animals instinctively freeze
  • when threatened by predators [?] do so reduces their visibility
    • to predators
      • that rely primarily on detecting movement
        • to locate their targets

[?] = what varies:

1. , to
2. . To
3. . As to
4. to

Understanding the Meaning

Let's start reading from the beginning:

'Many prey animals instinctively freeze when threatened by predators'

• This tells us that when prey animals sense danger from predators, their natural response is to freeze - to stop moving.

Now we've reached the blank. Let's look at our choices:

• They differ in punctuation (comma, period, or nothing)
• And in what word comes next: 'to,' 'To,' or 'As to'

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

The sentence continues: 'do so reduces their visibility to predators that rely primarily on detecting movement to locate their targets.'

Now let's really understand what this is telling us:

• 'to do so'
  • 'Do so' means 'do that action' - referring back to freezing
  • So 'to do so' = 'to freeze'

• 'to do so reduces their visibility'
  • This is explaining that freezing (doing so) has an effect:
  • it reduces how visible the prey animals are

• 'to predators that rely primarily on detecting movement'
  • The prey becomes less visible specifically to predators who hunt by spotting movement
  • These predators depend on seeing things move to find their targets

So the complete picture is:

• Prey animals freeze when threatened, and freezing reduces their visibility to predators who hunt by detecting movement.

What do we notice about the structure here?

• First part: 'Many prey animals instinctively freeze when threatened by predators'
  • This is a complete thought that could stand alone as a sentence
  • Subject: 'Many prey animals'
  • Verb: 'freeze'

• Second part: 'To do so reduces their visibility to predators that rely primarily on detecting movement to locate their targets'
  • This is also a complete thought that could stand alone
  • Subject: 'To do so' (the action of freezing)
  • Verb: 'reduces'
  • What gets reduced: 'their visibility'

We have two complete sentences here - two separate thoughts that can each stand on their own. When you have two complete sentences, you need to separate them properly with a period (or semicolon, or comma + conjunction, but those aren't options here).

So we need Choice B: predators. To - the period separates the two complete sentences, and 'To' starts the new sentence.

GRAMMAR CONCEPT APPLIED

Separating Complete Sentences with Proper Punctuation

When you have two complete sentences (each with its own subject and verb that express complete thoughts), you must separate them properly. You cannot simply run them together or connect them with just a comma.

Complete sentence 1: The scientists discovered a new species.
Complete sentence 2: The finding surprised the research community.

CORRECT ways to handle this:

• Two separate sentences: The scientists discovered a new species. The finding surprised the research community.
• Semicolon: The scientists discovered a new species; the finding surprised the research community.
• Comma + coordinating conjunction: The scientists discovered a new species, and the finding surprised the research community.

INCORRECT ways:

• Comma splice: The scientists discovered a new species, the finding surprised the research community.
• Run-on (no punctuation): The scientists discovered a new species the finding surprised the research community.

In this question:

• First complete sentence: 'Many prey animals instinctively freeze when threatened by predators'
• Second complete sentence: 'To do so reduces their visibility to predators that rely primarily on detecting movement to locate their targets'
• These must be separated with a period (making them two distinct sentences)

Note: An infinitive phrase like 'To do so' can serve as the subject of a sentence (this is called an infinitive as subject in grammar terms), which is why the second part is a complete sentence.