For decades, astronomers could only theorize about the existence of black holes through indirect observations of their gravitational effects. Recent...

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

• For decades,
  • astronomers
    • could only theorize about the existence of black holes
      • through indirect observations
        • of their gravitational effects.
• Recent technological breakthroughs
  • have changed that[?, though?]
    • the Event Horizon Telescope
      • captured the first direct image of a black hole
        • in 2019.

Understanding the Meaning

Let's read from the beginning to understand what this passage is telling us.

The first sentence sets up the historical situation:

• 'For decades, astronomers could only theorize about the existence of black holes through indirect observations of their gravitational effects.'
  • Astronomers couldn't actually see black holes directly
  • They could only make theories based on observing gravitational effects

Now the second sentence begins:

• 'Recent technological breakthroughs have changed that'
  • 'That' refers to the old limitation
  • Technology has changed the situation - astronomers can now do more than just theorize

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

• All choices have 'that' followed by 'though'
• What varies is the punctuation:
  • Comma or no comma after 'that'
  • Different punctuation after 'though' (comma, colon, or both)

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

• 'the Event Horizon Telescope captured the first direct image of a black hole in 2019'
  • This gives us a specific, concrete example
  • It shows exactly HOW the breakthroughs changed things

Now let's understand what we have structurally:

• 'Recent technological breakthroughs have changed that'
  • This is a complete thought on its own
• 'the Event Horizon Telescope captured the first direct image of a black hole in 2019'
  • This is also a complete thought
  • But it's providing a specific example of the first part

What do we notice about the structure here?

• We have two complete thoughts where the second one gives a specific example of the first
  • This is exactly when we use a colon - to introduce an explanation or example
• The word 'though' in this context works as a transitional element
  • It adds a conversational tone, like 'here's what I mean'
  • Transitional elements need to be set off with commas

So we need: a comma after 'that' to set off 'though,' then a colon after 'though' to introduce the specific example.

The correct answer is B: 'that, though:'

GRAMMAR CONCEPT APPLIED

Using Colons to Introduce Explanations with Transitional Elements

When you have two complete thoughts and the second one provides a specific example or explanation of the first, you can use a colon to connect them. If you include a transitional word (like 'though,' 'namely,' or 'specifically') before the colon, that transitional element needs to be set off with a comma:

Pattern: Complete thought + comma + transitional word + colon + explanatory example

Example 1:

• Without transition: 'The experiment confirmed our hypothesis: the bacteria grew faster in warmer temperatures.'
• With transition: 'The experiment confirmed our hypothesis, specifically: the bacteria grew faster in warmer temperatures.'

Example 2:

• 'The museum acquired several new pieces, though: a Monet painting arrived just yesterday.'
  • 'The museum acquired several new pieces, though' = complete thought
  • comma sets off 'though'
  • colon introduces the specific example
  • 'a Monet painting arrived just yesterday' = the specific example

In our question:

• 'Recent technological breakthroughs have changed that, though: the Event Horizon Telescope captured the first direct image of a black hole in 2019.'
  • First part: complete thought about change
  • 'though' adds conversational transition
  • Colon introduces the concrete example of that change

The key is recognizing when the second clause provides an example or explanation (use a colon) and properly punctuating any transitional elements (use commas).