As British scientist Peter Whibberley has observed, 'the Earth is not a very good timekeeper.' Earth's slightly irregular rotation rate...

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

• As British scientist Peter Whibberley has observed,
  • 'the Earth is not a very good timekeeper.'

• Earth's slightly irregular rotation rate
  • means that measurements of time
    • must be periodically adjusted.

• Specifically,
  • an extra 'leap second'
    • (the 86,401st second of the day)
  • is added [?]
  • whenever time based on the planet's rotation
    • lags a full nine-tenths of a second behind time
      • kept by precise atomic clocks.

Understanding the Meaning

The passage starts with some background context:

• A scientist observes that Earth isn't a perfect timekeeper
• Earth's rotation isn't perfectly regular
• So we need to adjust our time measurements sometimes

Now we get to the specific example:

• An extra 'leap second' (the 86,401st second of the day) is added [?]

This is where we have the blank.

Let's look at the choices:

• They're asking us whether we need punctuation after "added" – a comma, a semicolon, a period, or nothing at all.

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

The sentence continues:

• "whenever time based on the planet's rotation lags a full nine-tenths of a second behind time kept by precise atomic clocks"

Let's break this down:

• "whenever" tells us WHEN this leap second gets added
• It's added when the time based on Earth's rotation falls behind atomic clock time by nine-tenths of a second

So the complete picture is:

• A leap second is added whenever (at whatever time) the Earth-based time lags behind atomic clock time by 0.9 seconds.

What do we notice about the structure here?

• We have a complete statement: "an extra leap second is added"
  • This could stand alone as a sentence – it has a subject and verb and expresses a complete thought.

• Then we have "whenever time based on the planet's rotation lags..."
  • This part starts with "whenever" – a word that introduces a dependent, conditional element
  • It's telling us the condition or timing for when the leap second is added
  • This part CANNOT stand alone – it needs the main statement to complete its meaning

• The "whenever" clause flows directly from the main statement
  • It's providing essential information about when the action happens
  • The word "whenever" itself signals this relationship

When a dependent clause like this comes after the main statement and starts with a word like "whenever," we don't need any punctuation between them. The connecting word does the job of showing the relationship.

So we need: no punctuation – the correct answer is D.

GRAMMAR CONCEPT APPLIED

Connecting Dependent Clauses That Follow Main Statements

When you have a complete statement followed by a dependent clause (a clause introduced by words like "whenever," "because," "if," "when," "although," "since," etc. – called subordinating conjunctions in grammar terms), you typically don't need punctuation between them:

Pattern:

• Main statement + whenever/because/if/when clause = NO punctuation needed

Examples:

1. With "whenever":
   • Main statement: "The alarm sounds"
   • Dependent clause: "whenever someone opens the door"
   • Combined: "The alarm sounds whenever someone opens the door" ✓
2. With "because":
   • Main statement: "She stayed home"
   • Dependent clause: "because she wasn't feeling well"
   • Combined: "She stayed home because she wasn't feeling well" ✓
3. With "if":
   • Main statement: "Call me"
   • Dependent clause: "if you need help"
   • Combined: "Call me if you need help" ✓

In our question:

• Main statement: "an extra 'leap second' is added"
• Dependent clause: "whenever time based on the planet's rotation lags..."
• Combined correctly: "is added whenever time..."

Important note: The rule is DIFFERENT when the order is reversed. If the dependent clause comes FIRST, you DO need a comma:

• "Whenever time lags behind, a leap second is added" ✓