Coral reefs in the Caribbean have developed remarkable adaptations over millions of years. Through _____ complex symbiotic relationships with algae...

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

• Coral reefs in the Caribbean
• have developed remarkable adaptations
• over millions of years.

• Through [?] complex symbiotic relationships
• with algae and other marine organisms,
• the reefs
• support an extraordinary diversity of marine life
• despite nutrient-poor waters.

Understanding the Meaning

Let's start from the beginning:

'Coral reefs in the Caribbean have developed remarkable adaptations over millions of years.'

So we're learning about coral reefs (note: this is plural - multiple reefs) in the Caribbean, and they've developed some impressive adaptations over a very long time.

Now the second sentence begins:
'Through _____ complex symbiotic relationships with algae and other marine organisms...'

Here's where we need to fill in the blank. Let's look at our choices:

• A. its
• B. their
• C. it's
• D. they're

The choices are telling us we need to decide between:

• Possessive forms (its, their) - showing ownership or belonging
• Contractions (it's = it is, they're = they are)

Now, what do we need here? Let's think about what makes sense:

• We're talking about relationships - but whose relationships?
• The relationships belong to the coral reefs we just mentioned
• We need a word that shows possession (the reefs' relationships)
• "Coral reefs" is plural
• So we need a plural possessive: their

Let's continue reading to see the complete picture:
'Through their complex symbiotic relationships with algae and other marine organisms, the reefs support an extraordinary diversity of marine life despite nutrient-poor waters.'

This makes perfect sense! The sentence is explaining:

• The reefs have these complex partnerships (symbiotic relationships) with algae and other sea creatures
• Through these relationships, the reefs can support lots of different marine life
• And they do this even though the water doesn't have many nutrients

What do we notice about the structure here?

• We have a pronoun that needs to refer back to "coral reefs"
• "Coral reefs" is the antecedent - the noun the pronoun points back to
• Since "coral reefs" is plural, we need a plural pronoun
• Since we're showing that the relationships belong to the reefs, we need a possessive form

So we need their - the plural possessive pronoun that correctly refers back to "coral reefs."

GRAMMAR CONCEPT APPLIED

Matching Pronouns to What They Refer To

When you use a pronoun, it needs to match (or "agree with") the noun it refers back to. This means two things:

1. Number agreement: If the noun is singular, use a singular pronoun. If the noun is plural, use a plural pronoun.
   • Singular: The reef has its own ecosystem
   • Plural: The reefs have their own ecosystems
2. Form matters: Choose between possessive forms and contractions based on meaning:
   • Possessive (showing ownership/belonging): its, their
     • "The coral uses its tentacles" (the tentacles belong to the coral)
   • Contractions (shortened verb phrases): it's, they're
     • "It's alive" means "It is alive"
     • "They're growing" means "They are growing"

In this question:

• The noun is "coral reefs" (plural)
• We need to show possession (whose relationships?)
• So we need "their" (plural possessive)

Quick tip: If you can replace the word with "it is" or "they are" and it makes sense, use the contraction. If not, you need the possessive form. Here, "Through they are complex relationships" doesn't work, so we know we need the possessive "their."