In Death Valley National Park's Racetrack Playa, a flat, dry lakebed, are 162 rocks—some weighing less than a pound but...

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

• In Death Valley National Park's Racetrack Playa,
• a flat, dry lakebed,
• are 162 rocks—
• some weighing less than a pound but others almost 700 pounds—
• that move periodically from place to place,
• seemingly of their own volition.

• Racetrack-like trails
• in the (?) sediment
• mark the (?) mysterious migration.

Understanding the Meaning

Let's start with the first sentence to understand the context:

In Death Valley National Park's Racetrack Playa,

• which is described as "a flat, dry lakebed,"

are 162 rocks

• some weighing less than a pound but others almost 700 pounds
• that move periodically from place to place, seemingly of their own volition.

So we're learning about a fascinating natural phenomenon:

• There's a place called Racetrack Playa (a dry lakebed)
• It has 162 rocks that mysteriously move on their own

Now let's look at the second sentence where we have our blank:

"Racetrack-like trails in the ______ mysterious migration."

This is where we need to fill in the blank. Let's look at our choices to see what we're deciding:

• We need to decide how to write "playa" (in relation to sediment)
• We also need to decide how to write "rocks" (in relation to migration)

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

"Racetrack-like trails in the [playa's/playas/playas'] sediment mark the [rock's/rocks/rocks'] mysterious migration."

Now let's understand what this is telling us:

• "Racetrack-like trails in the sediment"
  • The moving rocks leave trails behind them
  • These trails are IN the sediment
  • More specifically, the sediment belongs to the playa

• "mark the mysterious migration"
  • These trails show evidence of the migration
  • Whose migration? The rocks' migration
  • We're talking about 162 rocks (plural)

What do we notice about the relationships here?

• First relationship: sediment and playa
  • The sediment belongs TO the playa
  • How many playas are we talking about? Just ONE (Racetrack Playa)
  • When one thing owns something → singular possessive → playa's

• Second relationship: migration and rocks
  • The migration belongs TO the rocks
  • How many rocks? 162 rocks (plural)
  • When multiple things own something → plural possessive → rocks'

So we need: playa's sediment mark the rocks' - Choice C.

GRAMMAR CONCEPT APPLIED

Forming Possessives: Singular vs. Plural

When showing that something belongs to someone or something else, you need to use possessive forms with apostrophes. The placement of the apostrophe depends on whether the owner is singular or plural:

For singular possessors (one thing owns something):

• Add 's to the singular noun
• Example: The dog's collar / The playa's sediment
• One dog owns the collar → dog's
• One playa has the sediment → playa's

For plural possessors (multiple things own something):

• If the plural already ends in 's', just add ' after the s
• Example: The dogs' collars / The rocks' migration
• Multiple dogs own the collars → dogs'
• Multiple rocks (162 of them) have a migration → rocks'

For plural possessors that don't end in 's':

• Add 's to the plural noun
• Example: The children's toys / The people's choice

In this question:

• "playa's sediment" → ONE playa owns the sediment → singular possessive → playa's
• "rocks' migration" → MANY rocks (162) share the migration → plural possessive → rocks'

The key is to ask yourself: (1) Is there possession happening? (2) Is the owner singular or plural? (3) Does the plural form end in 's'?