By analyzing ice cores from Greenland and Antarctica, a research team at Sweden's Lund University discovered evidence of a solar...

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

• By analyzing ice cores from Greenland and Antarctica,
  • a research team at Sweden's Lund University
    • discovered evidence of a solar storm
      • that occurred 9,200 years ago.
• Scientists had previously thought
  • the Sun
    • to be in a relatively 'quiet' phase
      • at that time [?]
  • the Lund team's finding
    • suggests otherwise.

Understanding the Meaning

The first sentence sets up the context:

• By analyzing ice cores from Greenland and Antarctica
  • a research team at Sweden's Lund University
  • discovered evidence of a solar storm
  • that occurred 9,200 years ago

So we have a discovery: evidence of an ancient solar storm from 9,200 years ago.

Now the second sentence begins:

• "Scientists had previously thought the Sun to be in a relatively 'quiet' phase at that time"
  • This tells us what scientists believed before this discovery
  • They thought the Sun wasn't very active 9,200 years ago

This is where we have the blank. Let's look at the choices - we're deciding between different punctuation options (comma, "but," both, or neither).

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

• "the Lund team's finding suggests otherwise"
  • The discovery contradicts what scientists previously thought
  • It suggests the Sun was NOT in a quiet phase

Now let's look at what we have structurally:

• "Scientists had previously thought the Sun to be in a relatively 'quiet' phase at that time"
  • This is a complete thought - it could stand alone as a sentence
  • Subject: Scientists
  • Verb: had previously thought
  • Complete idea expressed
• "the Lund team's finding suggests otherwise"
  • This is also a complete thought - it could stand alone too
  • Subject: the Lund team's finding
  • Verb: suggests
  • Complete idea expressed

What do we notice about the relationship here?

• We have two complete thoughts that could each be their own sentence
• But they're expressing contrasting ideas:
  • First part: what scientists thought before
  • Second part: what the finding actually suggests (the opposite)
• The word "otherwise" signals this contradiction

When we want to join two complete thoughts that have a contrasting relationship, we need both pieces:

• A comma (to signal the pause before the contrast)
• Plus the word "but" (to show the contrasting relationship)

So we need: time, but

The correct answer is Choice B.

GRAMMAR CONCEPT APPLIED

Joining Two Complete Thoughts with Comma + Coordinating Conjunction

When you have two complete thoughts (called independent clauses in grammar terms) that could each stand alone as sentences, and you want to join them into one sentence with a contrasting or connecting relationship, you use this pattern:

Pattern: [Complete thought], [but/and/or/so/yet] [complete thought]

Example 1 - Contrast:

• Two separate thoughts: "The experiment failed. The team learned valuable lessons."
• Joined with contrast: "The experiment failed, but the team learned valuable lessons."
  • First part: complete thought with subject "experiment" and verb "failed"
  • Second part: complete thought with subject "team" and verb "learned"
  • "But" shows the contrasting relationship

Example 2 - Addition:

• Two separate thoughts: "The temperature dropped. The hikers set up camp."
• Joined with addition: "The temperature dropped, and the hikers set up camp."
  • Comma + "and" joins two complete thoughts
  • Shows the ideas are connected/sequential

In this question:

• First complete thought: "Scientists had previously thought the Sun to be in a relatively 'quiet' phase at that time"
• Second complete thought: "the Lund team's finding suggests otherwise"
• Relationship: Contrast (what was thought vs. what was discovered)
• Solution: time, but (comma + "but" to show the contrasting relationship)

The comma gives readers a pause point before the contrast, and "but" signals that what comes next contradicts what came before.