When astronomers first detected the planet WASP-12b through their telescopes, they noted its extraordinary size and proximity to its star....

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

• Sentence 1:
  • When astronomers first detected the planet WASP-12b through their telescopes,
  • they noted
    • its extraordinary size and proximity to its star.
• Sentence 2:
  • Further spectroscopic analysis confirmed
    • the planet to be a gas giant [?]
      • atmosphere inflated to nearly twice Jupiter's radius
        • by the intense radiation from its host star.

Understanding the Meaning

The first sentence sets up the context:

• Astronomers detected a planet called WASP-12b through telescopes
• They noticed it was extraordinarily large and very close to its star

Now the second sentence tells us what further analysis revealed:

• Further spectroscopic analysis confirmed the planet to be a gas giant
  • This is telling us what kind of planet it is - a gas giant

This is where we have the blank.

Let's look at the choices:

• Choice A: semicolon + its (lowercase)
• Choice B: comma + and + its (lowercase)
• Choice C: comma + its (lowercase)
• Choice D: period + Its (capitalized)

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

The sentence continues: 'its atmosphere inflated to nearly twice Jupiter's radius by the intense radiation from its host star.'

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

• Its atmosphere inflated to nearly twice Jupiter's radius
  • This describes a specific characteristic of the gas giant
  • The planet's atmosphere has been inflated (puffed up) to nearly twice the size of Jupiter's radius
• By the intense radiation from its host star
  • This explains WHY the atmosphere is so inflated
  • The star's radiation is heating and expanding the atmosphere

So the complete picture is:

• Scientists confirmed this is a gas giant with an atmosphere that's been inflated to nearly twice Jupiter's radius because of intense radiation from its nearby star

What do we notice about the structure here?

• Further spectroscopic analysis confirmed the planet to be a gas giant
  • This is a complete thought that could stand alone as a sentence
• Its atmosphere inflated to nearly twice Jupiter's radius...
  • This is NOT a complete sentence
  • It's a noun ('its atmosphere') with a descriptive phrase ('inflated to...')
  • This phrase is providing extra descriptive information about the gas giant
  • Notice that 'inflated' here is describing the atmosphere, not acting as the main action verb of a new sentence

When we have a descriptive phrase like this that adds extra information about something in the main sentence, we connect it with a comma.

So we need: Choice C (giant, its)

GRAMMAR CONCEPT APPLIED

Using Commas to Connect Descriptive Phrases (Noun + Modifier)

When you have a descriptive phrase that provides additional information about something in your main sentence, you connect it with a comma. This often happens when you have a noun followed by a descriptive form (called a participial phrase or absolute phrase in grammar terms):

Pattern:

• Complete sentence + comma + noun + descriptive phrase

Example 1:

• Main sentence: "The team celebrated their victory"
• Added description: "their coach beaming with pride"
• Combined: "The team celebrated their victory, their coach beaming with pride."
• "Their coach" = noun
• "Beaming with pride" = descriptive phrase

Example 2:

• Main sentence: "The storm passed through the valley"
• Added description: "its winds reaching speeds of 100 mph"
• Combined: "The storm passed through the valley, its winds reaching speeds of 100 mph."
• "Its winds" = noun
• "Reaching speeds of 100 mph" = descriptive phrase

In our question:

• Main sentence: "Further spectroscopic analysis confirmed the planet to be a gas giant"
• Added description: "its atmosphere inflated to nearly twice Jupiter's radius by the intense radiation from its host star"
• Combined with comma: "...confirmed the planet to be a gas giant, its atmosphere inflated to nearly twice Jupiter's radius..."
• "Its atmosphere" = noun
• "Inflated to nearly twice Jupiter's radius..." = descriptive phrase

Key Point: Don't use a semicolon or period here because the second part isn't a complete sentence - it's descriptive information that depends on the main clause for its meaning.