The Alvarez theory, developed in 1980 by physicist Luis Walter Alvarez and his geologist son Walter Alvarez, maintained that the...

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 Alvarez theory,
  • developed in 1980 by physicist Luis Walter Alvarez
    • and his geologist son Walter Alvarez,
  • maintained that the secondary effects of an asteroid impact
    • caused many dinosaurs and other animals to die out [?]
      • it left unexplored
        • the question of whether unrelated volcanic activity
          • might have also contributed to the mass extinctions.

Where [?] = out / out, / out but / out, but

Understanding the Meaning

Let's start reading from the beginning:

The sentence tells us about the Alvarez theory:

• developed in 1980 by a physicist and his geologist son
• The theory maintained something specific

What did it maintain?

• that the secondary effects of an asteroid impact caused many dinosaurs and other animals to die out

This is where we have the blank.

Let's look at the choices:

• A: out but (no comma)
• B: out, but (comma + but)
• C: out (nothing)
• D: out, (comma only)

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

The sentence continues: "it left unexplored the question of whether unrelated volcanic activity might have also contributed to the mass extinctions."

Now let's understand what this second part is telling us:

• The theory (referred to as "it") left something unexplored
  • Specifically: the question about volcanic activity's role
  • The theory didn't address whether volcanoes might have also caused extinctions

So the complete picture is:

• First part: The theory maintained that asteroid impacts caused the extinctions
• Second part: BUT the theory didn't explore whether volcanic activity also played a role

What do we notice about the structure here?

• The first part is a complete thought:
  • "The Alvarez theory maintained that the secondary effects of an asteroid impact caused many dinosaurs and other animals to die out"
  • It has a subject (theory), a verb (maintained), and expresses a complete idea

• The second part is also a complete thought:
  • "it left unexplored the question of whether unrelated volcanic activity might have also contributed to the mass extinctions"
  • It has a subject (it), a verb (left), and expresses a complete idea

• These two complete thoughts have a contrasting relationship:
  • The theory said one thing BUT didn't address another thing
  • We need to show this contrast

When we're connecting two complete thoughts that contrast with each other, we need:

• A comma before the connecting word
• The word "but" to show the contrast

So we need: out, but (Choice B)

GRAMMAR CONCEPT APPLIED

Connecting Two Complete Thoughts with a Comma and Coordinating Conjunction

When you have two complete thoughts (called independent clauses in grammar terms) and you want to connect them while showing their relationship, you need both a comma AND a coordinating conjunction:

Pattern: Complete thought, [coordinating conjunction] complete thought

The coordinating conjunctions are: for, and, nor, but, or, yet, so (FANBOYS)

Example 1 - Showing contrast with "but":

• Complete thought 1: The study examined sleep patterns
• Complete thought 2: It didn't account for dietary factors
• Connected: The study examined sleep patterns, but it didn't account for dietary factors

Example 2 - Showing addition with "and":

• Complete thought 1: The museum opened a new wing
• Complete thought 2: It attracted record numbers of visitors
• Connected: The museum opened a new wing, and it attracted record numbers of visitors

In this question:

• Complete thought 1: "The Alvarez theory maintained that the secondary effects of an asteroid impact caused many dinosaurs and other animals to die out"
• Complete thought 2: "it left unexplored the question of whether unrelated volcanic activity might have also contributed to the mass extinctions"
• Relationship: Contrast (the theory addressed one thing BUT not another)
• Connected correctly: die out, but it left unexplored...

Key point: Both the comma AND the conjunction are necessary. The comma alone creates a comma splice, and the conjunction alone without a comma creates a run-on sentence.