When particles are suspended in liquid (like pollen in a water glass), they will zigzag randomly through the liquid and...

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

• When particles are suspended in liquid (like pollen in a water glass),
  • they will zigzag randomly through the liquid
    • and collide with one another in perpetuity.
• This type of random, continuous movement [?] is known as Brownian motion,
  • can be observed throughout the natural world.

Understanding the Meaning

The first sentence sets up the scenario:

• When particles are suspended in liquid (like pollen in a water glass)
  • We're talking about tiny particles floating in water or another liquid

• They will zigzag randomly through the liquid and collide with one another in perpetuity
  • These particles move in random, zigzag patterns
  • They keep bumping into each other
  • "In perpetuity" means this keeps happening continuously, forever

Now the second sentence connects to this:

• "This type of random, continuous..."
  • "This type" refers back to the movement described above

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

• They all include "movement"
• But they differ in punctuation: colon-which, comma-which, no punctuation-which, period-which

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

• "...is known as Brownian motion, can be observed throughout the natural world."

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

If I read this straight through with just "movement":

• "This type of random, continuous movement is known as Brownian motion, can be observed..."

Wait - what do we notice about the structure here?

• We seem to have TWO verb phrases:
  • "is known as Brownian motion"
  • "can be observed throughout the natural world"
• Both are trying to be the main verb, which creates a run-on sentence

But look - all our choices include "which." Let's see what that does:

• "This type of random, continuous movement, which is known as Brownian motion, can be observed throughout the natural world."

Now it makes sense! Here's what's happening:

• The SUBJECT is: "This type of random, continuous movement"

• The phrase "which is known as Brownian motion"
  • is giving us extra information - the scientific name for this movement
  • It's like a side note - "by the way, this is called Brownian motion"
  • This is not the main point of the sentence

• The MAIN VERB is: "can be observed throughout the natural world"
  • THIS is what the sentence is really telling us
  • This movement can be seen everywhere in nature

So the complete meaning is:

• This type of random, continuous movement (which, by the way, is called Brownian motion) can be observed throughout the natural world.

What do we notice about punctuation?

• The "which" clause is providing additional, non-essential information
  • We already know what movement we're talking about (the random, continuous type)
  • The "which" clause is just telling us its scientific name
• When a phrase provides extra information like this, it needs to be set off by commas on BOTH sides
  • One comma before "which" to open the extra information
  • One comma after "motion" to close it (this one is already there!)

So we need: movement, which

The correct answer is B.

GRAMMAR CONCEPT APPLIED

Using Commas with Non-Essential Descriptive Clauses

When you add extra information about something using "which," that information needs to be set off by commas on both sides (this type of clause is called a non-essential relative clause in grammar terms). Think of the commas like parentheses - they bracket off bonus information:

Pattern: Noun, which [extra information], verb

Example 1:

• Without the extra info: "The discovery changed scientific understanding."
• With extra info added: "The discovery, which was made in 1905, changed scientific understanding."
  • "which was made in 1905" = extra detail about when
  • Commas on both sides set it off

Example 2:

• Without the extra info: "My laptop needs repair."
• With extra info added: "My laptop, which I bought last year, needs repair."
  • "which I bought last year" = extra detail about purchase
  • Commas on both sides set it off

In our question:

• Subject: "This type of random, continuous movement"
• Extra info: "which is known as Brownian motion"
  • Just telling us the scientific name - bonus information
• Main verb: "can be observed throughout the natural world"
• Result: "movement, which is known as Brownian motion, can be observed..."

Key distinction:

• "Which" clauses (adding extra info) = need commas
• "That" clauses (essential to identify what you're talking about) = no commas