Astronomer Henrietta Swan Leavitt revolutionized cosmic distance measurement through her analysis of variable stars in the early twentieth century. He...

Sentence Structure

• Sentence 1:
  • Astronomer Henrietta Swan Leavitt
    • revolutionized cosmic distance measurement
      • through her analysis of variable stars
        • in the early twentieth century.
• Sentence 2:
  • Her discovery
    • established
      • that Cepheid variables could serve as reliable "standard candles"
        • for calculating astronomical distances [?] in the 1910s [?]
  • her breakthrough
    • enabled Edwin Hubble
      • to later demonstrate the universe's expansion.
• Where [?] represents the punctuation we need to determine around "in the 1910s"

Understanding the Meaning

The first sentence sets up the context:

• Henrietta Swan Leavitt was an astronomer who changed how we measure distances in space
• She did this through studying variable stars in the early 1900s

Now the second sentence tells us more:

• "Her discovery established that Cepheid variables could serve as reliable 'standard candles' for calculating astronomical distances"
  • Her work proved that these special stars (Cepheid variables) could be used as measuring tools for figuring out how far away things are in space

This is where we have the blank with the time reference "in the 1910s."

Let's look at what the choices are showing us:

• All choices include "distances" and "in the 1910s"
• What varies is the punctuation around this time phrase
• We need to continue reading to see what comes next and understand how these parts connect

The sentence continues:

• "her breakthrough enabled Edwin Hubble to later demonstrate the universe's expansion"
  • This tells us that Leavitt's work made it possible for Edwin Hubble (another astronomer) to prove the universe is expanding

Now let's understand what we have structurally:

• The first part: "Her discovery established that Cepheid variables could serve as reliable 'standard candles' for calculating astronomical distances"
  • This is a complete thought - it could stand alone as a sentence
  • It has a subject (Her discovery) and tells us what that discovery did
• The second part: "her breakthrough enabled Edwin Hubble to later demonstrate the universe's expansion"
  • This is ALSO a complete thought - it could stand alone too
  • It has a subject (her breakthrough) and tells us what that breakthrough enabled
• The time reference: "in the 1910s"
  • Looking at the meaning, this time phrase tells us WHEN her breakthrough enabled Hubble's work
  • It's not about when distances were calculated, but when Hubble was able to use her work

What do we notice about the structure here?

• We have two complete sentences - two independent thoughts that could each stand alone
• These can't just be joined with a comma (that would be a comma splice) or with no punctuation (that would be a run-on)
• We need strong punctuation to separate them - a semicolon works perfectly
• "In the 1910s" serves as an introductory time phrase for the second sentence
• Introductory phrases are set off with a comma

So we need: distances; in the 1910s,

• The semicolon separates the two complete thoughts
• The comma sets off the introductory time phrase for the second sentence

The correct answer is Choice D.

GRAMMAR CONCEPT APPLIED

Separating Complete Sentences and Using Introductory Phrases

When you have two complete sentences (called independent clauses in grammar terms) - each with its own subject and verb that could stand alone - you need strong punctuation to separate them. A semicolon is one way to do this:

Pattern for separating complete sentences:

• Complete Sentence 1; Complete Sentence 2
• Example: "The experiment was successful; the results exceeded our expectations."
• First complete sentence: "The experiment was successful"
• Second complete sentence: "the results exceeded our expectations"

Adding an introductory phrase to the second sentence:

When the second sentence begins with an introductory phrase (like a time reference), you use a semicolon to end the first sentence, then set off the introductory phrase with a comma:

• Complete Sentence 1; introductory phrase, rest of Complete Sentence 2
• Example: "The team finished their research; by 2020, they had published their findings."
• First complete sentence: "The team finished their research"
• Introductory time phrase: "by 2020"
• Rest of second sentence: "they had published their findings"

In our question:

• First complete sentence: "Her discovery established that Cepheid variables could serve as reliable 'standard candles' for calculating astronomical distances"
• Semicolon separates it from the next complete thought
• Introductory time phrase: "in the 1910s"
• Rest of second sentence: "her breakthrough enabled Edwin Hubble to later demonstrate the universe's expansion"

Therefore: "distances; in the 1910s, her breakthrough..."