In 2018, a team of researchers led by Dr. Caitlin Whalen compiled every available measurement of ocean mixing rates from...

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

• In 2018,
  • a team of researchers
    • led by Dr. Caitlin Whalen
  • compiled every available measurement
    • of ocean mixing rates
    • from the past two decades.

• With this novel data set,
  • the team
    • was able to determine
      • how current-driven mixing varies across regions [?]
      • and
      • what impact it has
        • on the distribution of heat and nutrients in the ocean.

Understanding the Meaning

The first sentence sets up the context:

• In 2018, a team led by Dr. Whalen compiled measurements
  • specifically measurements of ocean mixing rates
  • from the past two decades

Now the second sentence tells us what they did with this data:

• With this novel data set, the team was able to determine...

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

• regions, (with comma)
• regions: (with colon)
• regions; (with semicolon)
• regions (with no punctuation)

So we're deciding what punctuation, if any, should come after "regions."

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

The team was able to determine TWO things:

• First: "how current-driven mixing varies across regions"
  • This is asking about variation - how does the mixing change from place to place?

• Second: "what impact it has on the distribution of heat and nutrients in the ocean"
  • This is asking about the effect - what does this mixing actually do?

These two parts are connected by "and."

What do we notice about the structure here?

• The verb "determine" is followed by two parallel questions
  • Both are telling us what the team figured out
  • Both serve the same grammatical role - they're objects of "determine"

• These two parallel elements are joined by "and"
  • When we connect two parallel elements with "and," we don't put punctuation before the "and" (unless we have three or more items in a series)
  • The structure is: determine X and Y (no punctuation needed)

So we need no punctuation after "regions." The correct answer is Choice D.

GRAMMAR CONCEPT APPLIED

Connecting Parallel Elements with Coordinating Conjunctions

When two parallel grammatical elements (things serving the same function in a sentence) are joined by a coordinating conjunction like "and," we typically don't use punctuation before the conjunction:

Pattern: Verb + Object 1 and Object 2 (no punctuation)

Examples:

• She enjoys reading and writing.
  • "reading" and "writing" are parallel objects of "enjoys"
  • No punctuation before "and"

• The study examined how plants grow in shade and what nutrients they need.
  • "how plants grow in shade" and "what nutrients they need" are parallel noun clauses (called indirect questions in grammar terms)
  • Both are objects of "examined"
  • No punctuation before "and"

In our sentence:

• "determine how current-driven mixing varies across regions and what impact it has..."
• The two noun clauses are compound objects of "determine"
• They're parallel elements joined by "and"
• No punctuation is needed

Important exception: When you have THREE or more parallel elements (a series), you do use commas: "She enjoys reading, writing, and painting."