Official measurements of the Mississippi River's length vary: according to the US Geologic Survey, the river is 2,300 miles long,...

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:

• Official measurements of the Mississippi River's length vary:
  • according to the US Geologic Survey,
    • the river is 2,300 miles long,
  • whereas the Environmental Protection Agency
    • records its length as 2,320 miles.

Sentence 2:

• This disparity can be explained in part
  • by the fact that
    • rivers such as the Mississippi expand and contract
      • as [?] accumulate(s) sediment.

Understanding the Meaning

The first sentence sets up the context:

• Different official sources give different measurements for how long the Mississippi River is.
  • The US Geologic Survey says 2,300 miles
  • The EPA says 2,320 miles

Now the second sentence explains why these measurements might differ:

• 'This disparity can be explained in part by the fact that...'
  • The difference in measurements can be partly explained by something

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

• A: they accumulate
• B: one accumulates
• C: it accumulates
• D: we accumulate

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

The sentence tells us that:

• 'rivers such as the Mississippi expand and contract as ____ sediment'

Now let's understand what this is telling us:

• 'Rivers such as the Mississippi'
  • We're talking about multiple rivers - rivers like the Mississippi
  • This is a plural noun - more than one river

• 'Expand and contract'
  • Rivers change size - they get bigger and smaller

• 'As ____ sediment'
  • This is describing WHEN or WHY the rivers expand and contract
  • It happens as they accumulate (gather, collect) sediment

What do we notice about the structure here?

• The word we need in the blank must refer back to 'rivers such as the Mississippi'
  • That's what's doing the accumulating of sediment

• Since 'rivers such as the Mississippi' is plural (multiple rivers),
  • We need a plural pronoun to refer back to it

• 'They' is the pronoun that refers to plural nouns
  • 'They accumulate' matches with the plural 'rivers'

So the correct answer is A: they accumulate.

The complete meaning: River measurements vary partly because rivers like the Mississippi expand and contract as they accumulate sediment - the rivers themselves are constantly changing size as they collect sediment.

Grammar Concept Applied

Pronouns Must Match Their Antecedents in Number

When you use a pronoun, it must agree with the noun it's referring back to (called the antecedent in grammar terms). If the antecedent is plural, you need a plural pronoun. If it's singular, you need a singular pronoun.

The Pattern:

• Plural antecedent → Plural pronoun
  • "The students completed their homework"
  • "students" (plural) → "their" (plural)

• Singular antecedent → Singular pronoun
  • "The student completed his homework"
  • "student" (singular) → "his" (singular)

In this question:

• Antecedent: "rivers such as the Mississippi" (plural)
• Pronoun needed: "they" (plural)
• "Rivers... expand and contract as they accumulate sediment"

Common trap: Sometimes the antecedent appears several words before the pronoun, making it easy to lose track of whether it's singular or plural. Always trace back to find what noun the pronoun is replacing.