The fine, powdery substance that covers the Moon's surface is called regolith. Because regolith is both readily available and high...

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

• The fine, powdery substance
• that covers the Moon's surface
• is called regolith.

• Because regolith is both readily available
• and high in oxygen content [?]
• scientists have wondered
• whether it could be used as a potential source of oxygen
• for future lunar settlements.

Where [?] = what varies:

• A: and
• B: , (comma only)
• C: (nothing)
• D: , and

Understanding the Meaning

First sentence is straightforward:

• The Moon's surface is covered with a fine, powdery substance called regolith.

Now the second sentence gives us information about this regolith:

• "Because regolith is both readily available and high in oxygen content____"
  • It tells us two key properties:
    • readily available (there's a lot of it)
    • high in oxygen content (it contains a lot of oxygen)

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

• Some include just a comma (B)
• Some include "and" (A, D)
• Some include both comma and "and" (D)
• One includes neither (C)

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

• "scientists have wondered whether it could be used as a potential source of oxygen for future lunar settlements."
  • So scientists are thinking: maybe we could use this regolith to get oxygen for future Moon bases!

Now let's understand the complete picture:

• The sentence is explaining that BECAUSE regolith has these two properties (available and oxygen-rich), scientists have been wondering if they could use it for oxygen.

What do we notice about the structure here?

• The first part: "Because regolith is both readily available and high in oxygen content"
  • This is a dependent clause - it starts with "Because" and can't stand alone as a complete sentence
  • The "both...and" within this clause connects "readily available" and "high in oxygen" - that coordination is complete

• The second part: "scientists have wondered whether it could be used..."
  • This is the main clause - a complete thought that could stand on its own

• When a dependent clause introduces a sentence (comes before the main clause), we need a comma to separate it from the main clause
  • This comma shows where the introductory information ends and the main point begins

• We do NOT need "and" here because:
  • "And" is used to coordinate equal, parallel elements
  • But these two clauses aren't equal - one is dependent (the "Because" part) and one is independent (the main point)
  • The "Because" already establishes the relationship between them

So we need just a comma after "content" - nothing else.

The correct answer is B: "content,"

GRAMMAR CONCEPT APPLIED

Using Commas After Introductory Dependent Clauses

When you begin a sentence with a dependent clause (also called a subordinate clause in grammar terms), you need to use a comma to separate it from the main independent clause that follows.

How to recognize this pattern:

A dependent clause often starts with words like:

• Because, Although, While, When, If, Since, Unless, etc.

These clauses cannot stand alone as complete sentences - they depend on the main clause to complete the thought.

The rule:
Dependent clause at the start + COMMA + Independent clause

Examples:

• "Because it was raining, we canceled the picnic."
  • Dependent clause: "Because it was raining"
  • Independent clause: "we canceled the picnic"
  • Comma separates them

• "Although the test was difficult, most students passed."
  • Dependent clause: "Although the test was difficult"
  • Independent clause: "most students passed"
  • Comma separates them

In our question:

• "Because regolith is both readily available and high in oxygen content, scientists have wondered..."
  • Dependent clause: "Because regolith is both readily available and high in oxygen content"
  • Independent clause: "scientists have wondered whether..."
  • Comma after "content" separates them

Important note: Don't add "and" in this situation. The subordinating word (like "Because") already establishes the relationship between the clauses. Adding "and" would incorrectly try to coordinate them instead.