The first exoplanet in the habitable zone was conclusively identified _____ astronomers at the European Southern Observatory, opening new possibilitie...

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

• The first exoplanet in the habitable zone
• was conclusively identified by(?) astronomers at the European Southern Observatory,
• opening new possibilities for discovering extraterrestrial life.

Understanding the Meaning

Let's start from the beginning and understand what this sentence is telling us:

The sentence is about a significant discovery:

• 'The first exoplanet in the habitable zone'
  • An exoplanet is a planet outside our solar system
  • The 'habitable zone' means the area where life could potentially exist
  • This is telling us about the FIRST such planet discovered

• 'was conclusively identified'
  • Scientists definitively found and confirmed this planet

Now here's where we have the blank - the sentence continues with 'by(?)':

• We need to see what comes next to understand what works here

Let's look at our choices:

• The options are about what punctuation (if any) should come after "by"
  • by (no punctuation)
  • by, (with a comma)
  • by: (with a colon)
  • by; (with a semicolon)

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

• 'by astronomers at the European Southern Observatory'
  • This tells us WHO made this identification
  • The astronomers work at a specific place - the European Southern Observatory

• 'opening new possibilities for discovering extraterrestrial life'
  • This discovery created new opportunities to find alien life

What do we notice about the structure here?

• The phrase "by astronomers at the European Southern Observatory" is one continuous unit
  • "By" is connecting us to WHO did the identifying
  • "Astronomers at the European Southern Observatory" tells us the specific people

• These parts need to flow together smoothly
  • "By" + "astronomers" work as a pair - they're describing the doer of the action
  • Just like you'd say "identified by scientists" or "written by the author"
  • There's no natural pause or break between "by" and "astronomers"

So we need no punctuation after "by" - it should flow directly into "astronomers."

The correct answer is A (by).

GRAMMAR CONCEPT APPLIED

Keeping Prepositions Connected to Their Objects

When you use a preposition (words like "by," "in," "on," "with," "from," etc.) to show a relationship, the preposition needs to connect directly to its object (the noun or noun phrase that follows) without any punctuation separating them:

Pattern:

• Preposition + Object (+ any descriptors of that object) = one continuous unit
• No punctuation should interrupt this flow

Examples:

• "The book was written by the famous author from Ireland"
  • by = preposition
  • the famous author = object
  • from Ireland = descriptor
  • All flow together without punctuation

• "The discovery was made in the laboratory at Stanford University"
  • in = preposition
  • the laboratory = object
  • at Stanford University = descriptor
  • No punctuation breaks up this phrase

In this question:

• "identified by astronomers at the European Southern Observatory"
  • by = preposition
  • astronomers = object
  • at the European Southern Observatory = descriptor
  • These must stay connected: "by astronomers" (no punctuation between them)

This is called a prepositional phrase in grammar terms, and it functions as one cohesive unit in a sentence - so don't break it apart with punctuation!