Scientists have extensively studied the Great Barrier Reef's ecosystem. The reef provides habitat for thousands of marine _____ its health...

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

• Scientists
  • have extensively studied
    • the Great Barrier Reef's ecosystem.

• The reef
  • provides
    • habitat
      • for thousands of marine species (?)

• its health
  • has declined
    • significantly
      • in recent decades.

Understanding the Meaning

Let's start reading from the beginning:

"Scientists have extensively studied the Great Barrier Reef's ecosystem."

• This tells us that scientists have done a lot of research on how the Great Barrier Reef ecosystem works.

Now the second sentence starts:

"The reef provides habitat for thousands of marine species"

• This is telling us something positive about the reef –
  • it's home to thousands of different kinds of marine animals and plants.

This is where we have the blank.

Let's look at the choices:

• A. species, but
• B. species but:
• C. species but,
• D. species, but,

So we're deciding on the punctuation around the word "but."

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

"its health has declined significantly in recent decades."

• This tells us something negative –
  • the reef's health has gotten much worse over the past few decades.

So the complete picture is:

• The sentence is presenting two contrasting ideas:
  • First: the reef provides habitat (positive)
  • Second: BUT its health has declined (negative)
• The word "but" is showing this contrast.

What do we notice about the structure here?

• "The reef provides habitat for thousands of marine species"
  • This is a complete thought that could stand alone as its own sentence.
  • It has a subject (the reef) and a verb (provides).

• "its health has declined significantly in recent decades"
  • This is also a complete thought that could stand alone.
  • It has a subject (its health) and a verb (has declined).

• We're using "but" to connect these two complete thoughts and show the contrast between them.

When we connect two complete thoughts with a word like "but," we need a comma BEFORE the connecting word, not after it.

So we need: species, but

The correct answer is Choice A.

GRAMMAR CONCEPT APPLIED

Using Commas with Connecting Words to Join Complete Thoughts

When you want to connect two complete sentences using a connecting word like "but" (or "and," "or," "so," "yet" - called coordinating conjunctions in grammar terms), you need a comma BEFORE the connecting word:

Pattern: [Complete thought], [connecting word] [complete thought].

Example 1:

• Complete thought #1: "The study found promising results"
• Complete thought #2: "more research is needed"
• Connected properly: "The study found promising results, but more research is needed."

Example 2:

• Complete thought #1: "The temperature dropped below freezing"
• Complete thought #2: "the plants survived"
• Connected properly: "The temperature dropped below freezing, but the plants survived."

In our question:

• Complete thought #1: "The reef provides habitat for thousands of marine species"
• Complete thought #2: "its health has declined significantly in recent decades"
• Connected properly: "The reef provides habitat for thousands of marine species, but its health has declined significantly in recent decades."

The comma goes BEFORE "but" to signal that we're connecting two equally important complete thoughts, with "but" showing the contrast between them.