Marine biologists have observed increased coral bleaching in previously healthy reef systems, indicating rising ocean temperatures. Their assessments ...

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

• Marine biologists have observed increased coral bleaching
  • in previously healthy reef systems,
  • indicating rising ocean temperatures.

• Their assessments are concerning[?] natural temperature fluctuations
  • have occurred cyclically
    • throughout the reef's documented history
  • and may account for some observed changes.

Where [?] represents:

• A: ; though,
• B: , though;
• C: ; though
• D: , though,

Understanding the Meaning

Let's start from the beginning:

The first sentence tells us:

• Marine biologists observed increased coral bleaching in healthy reef systems
  • This bleaching indicates rising ocean temperatures

So we have a concerning observation about coral reefs and warming oceans.

Now the second sentence begins:

• "Their assessments are concerning"

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

• We're deciding between semicolons and commas
• We're also deciding whether "though" needs a comma after it

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

• "natural temperature fluctuations have occurred cyclically throughout the reef's documented history and may account for some observed changes"

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

• First part: "Their assessments are concerning"
  • The biologists' conclusions about rising temperatures are worrying

• The word "though" signals a contrast is coming
  • "though" means "however" or "but" - it introduces an opposing idea

• Second part: "natural temperature fluctuations have occurred cyclically throughout the reef's documented history and may account for some observed changes"
  • This is saying: BUT natural temperature changes have happened in cycles throughout history
  • And these natural cycles might explain some of what we're seeing

So the complete picture is:

• The assessments are concerning... HOWEVER, natural fluctuations have occurred historically and might explain some changes

What do we notice about the structure here?

• "Their assessments are concerning"
  • This is a complete thought - it has a subject and verb and could stand alone as a sentence

• "natural temperature fluctuations have occurred cyclically throughout the reef's documented history and may account for some observed changes"
  • This is ALSO a complete thought - it has its own subject ("natural temperature fluctuations") and verb ("have occurred")

• "Though" is connecting these two complete thoughts and showing contrast between them

When we connect two complete thoughts using a transitional word like "though," we need:

• A semicolon BEFORE the transitional word (to properly separate the two complete thoughts)
• A comma AFTER the transitional word (standard punctuation for these transition words)

So we need: concerning; though,

The correct answer is Choice A.

Grammar Concept Applied

Connecting Two Complete Thoughts with Transitional Words

When you want to connect two complete thoughts (sentences that could stand alone) using a transitional word like "though," "however," "therefore," or "moreover," you need specific punctuation:

Pattern: [Complete thought]; [transitional word], [complete thought].

Example 1:

• The experiment succeeded; however, the results were unexpected.
• "The experiment succeeded" = complete thought
• "however" = transitional word showing contrast
• "the results were unexpected" = complete thought

Example 2:

• The data supports the hypothesis; therefore, we can proceed with publication.
• "The data supports the hypothesis" = complete thought
• "therefore" = transitional word showing logical consequence
• "we can proceed with publication" = complete thought

In our question:

• Their assessments are concerning; though, natural temperature fluctuations have occurred...
• "Their assessments are concerning" = complete thought
• "though" = transitional word showing contrast
• "natural temperature fluctuations have occurred..." = complete thought

Why this punctuation?

• The semicolon properly separates two complete thoughts (a comma alone would create a run-on sentence)
• The comma after the transitional word is standard convention for these connecting words (sometimes called conjunctive adverbs in grammar terms)