Native to the Amazon rainforest, the glass frog is a small amphibian _____ by its translucent skin, which allows observers...

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

• Native to the Amazon rainforest,
• the glass frog
• is a small amphibian (?) by its translucent skin,
  • which allows observers to see its internal organs.

Understanding the Meaning

Let's start reading from the beginning:

'Native to the Amazon rainforest,'

• This opening phrase tells us where the glass frog lives originally.

'the glass frog'

• Here's our subject – the creature we're learning about.

'is a small amphibian'

• And here we have the main information: what the glass frog IS.
• Notice: 'is' is our main verb here – it's telling us the primary fact about the glass frog.

Now we reach the blank: '_____ by its translucent skin'

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

• A: is characterized
• B: has been characterized
• C: will be characterized
• D: characterized

What are these choices showing us?

• They're all forms of the verb "characterized"
• Options A, B, and C are all complete verb forms (with helping verbs)
• Option D is just the participle form without a helping verb

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

The sentence continues:

• 'by its translucent skin, which allows observers to see its internal organs'
• This is describing what makes the glass frog special – its see-through skin.

So the complete picture is:

• We're defining what the glass frog is (a small amphibian)
• AND we're adding extra information about what makes it distinctive (its translucent skin)

What do we notice about the structure here?

• We already have our main verb: 'is'
  • The sentence tells us: the glass frog IS a small amphibian
  • That's the core statement

• So what role does the blank play?
  • It's not making a new statement
  • It's adding descriptive information about "a small amphibian"
  • It's like adding a detail: "a small amphibian [with this particular feature]"

• When we already have a main verb and we want to ADD description, we need a participial phrase – not another complete verb
  • If we used another complete verb form (like "is characterized"), we'd have two main verbs competing with each other
  • That creates a grammatically incorrect structure

So we need: characterized

This gives us: "the glass frog is a small amphibian characterized by its translucent skin"

• Main statement: is a small amphibian
• Additional description: characterized by its translucent skin

The correct answer is D.

GRAMMAR CONCEPT APPLIED

Using Participial Phrases vs. Main Verbs

When a sentence already has a main verb expressing the primary action or state, additional information often needs to be added using participial phrases (verb forms without helping verbs that function as modifiers) rather than adding more complete verb forms:

Structure:

• Main statement with verb: The glass frog is a small amphibian
• Adding description with participial phrase: The glass frog is a small amphibian characterized by its translucent skin
• Main verb: "is" (tells us what the frog IS)
• Participial phrase: "characterized by its translucent skin" (adds descriptive detail about the amphibian)

Compare with incorrect structure:

• The glass frog is a small amphibian is characterized by its translucent skin
• This has two main verbs ("is" and "is characterized") with no proper connector
• Creates a run-on or grammatically incomplete structure

Another example:

• The scientist presented a theory supported by extensive research
• Main verb: "presented"
• Participial phrase: "supported by extensive research" (describes the theory)
• The scientist presented a theory is supported by extensive research
• Two main verbs without proper connection

In this question, "characterized" (without helping verbs) functions as a participial phrase that modifies "amphibian," while "is" remains the only main verb carrying the primary meaning of the sentence.