When external forces are applied to common glass made from silicates, energy builds up around minuscule defects in the material,...

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

Sentence 1:

• When external forces are applied to common glass made from silicates,
  • energy builds up around minuscule defects in the material,
  • resulting in fractures.

Sentence 2:

• Recently,
  • engineer Erkka Frankberg of Tampere University in Finland
    • used the chemical compound [?] aluminum oxide [?] to make a glassy solid that can withstand higher strain than silicate glass can before fracturing.

Where [?] = decision about comma placement:

• Choice A: compound, aluminum oxide
• Choice B: compound aluminum oxide,
• Choice C: compound, aluminum oxide,
• Choice D: compound aluminum oxide

Understanding the Meaning

The first sentence sets up the context:

• When forces are applied to regular silicate glass,
  • energy builds up around tiny defects,
  • causing the glass to crack.

Now let's read the second sentence:

• Recently, an engineer named Erkka Frankberg from Tampere University in Finland
  • used a particular chemical...

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

• They're asking us whether to put commas around 'aluminum oxide'
• Choice A: comma before only
• Choice B: comma after only
• Choice C: commas on both sides
• Choice D: no commas at all

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

The complete phrase is:

• 'used the chemical compound aluminum oxide to make a glassy solid that can withstand higher strain than silicate glass can before fracturing'

Now let's understand the relationship between these words:

• 'the chemical compound'
  • This is a general term - there are thousands of different chemical compounds
  • By itself, we don't know which one is being discussed

• 'aluminum oxide'
  • This tells us the SPECIFIC compound being used
  • It identifies which compound the engineer chose

What do we notice about this structure?

• 'aluminum oxide' is essential information
  • Without it, we wouldn't know which chemical compound was used
  • It's not just adding extra details - it's telling us which compound
  • This is identifying information that completes the meaning

• When information is essential to identify what you're talking about,
  • you don't separate it with commas
  • it needs to flow directly as part of the noun phrase

Think of it like this:

• 'my friend Sarah' (no commas - Sarah identifies which friend)
• vs. 'my only sister, Maria' (commas - 'only' already identifies there's just one, so Maria is extra info)

Here, 'aluminum oxide' works like 'Sarah' - it's identifying which compound.

So we need: compound aluminum oxide (no commas)

The correct answer is Choice D.

GRAMMAR CONCEPT APPLIED

Using Commas with Essential vs. Non-Essential Information

When a word or phrase identifies what you're talking about (essential information), you don't use commas. When it adds extra details about something already identified (non-essential information - called a non-restrictive modifier in grammar terms), you use commas to set it off:

Essential - No Commas:

• The chemical compound aluminum oxide was used
  • 'aluminum oxide' identifies which compound
  • Without it, we wouldn't know which one

• Students who study regularly tend to do well
  • 'who study regularly' identifies which students
  • We're not talking about all students, just the ones who study regularly

Non-Essential - Commas:

• Aluminum oxide, a common chemical compound, was used
  • We already know what substance (aluminum oxide)
  • 'a common chemical compound' just adds extra information

• My oldest sister, who lives in Boston, is visiting
  • 'oldest' already identifies which sister
  • Where she lives is just extra detail

In our question:

• 'the chemical compound aluminum oxide' - no commas needed
  • 'aluminum oxide' is essential because it specifies which compound
  • The sentence needs this information to be complete and meaningful