Planetary scientist Briony Horgan and her colleagues have determined that as much as 25 percent of the sand on Mars...

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

• Planetary scientist Briony Horgan and her colleagues
  • have determined
    • that as much as 25 percent of the sand on Mars
      • is composed of impact spherules.
• These spherical bits of glass
  • form
    • when asteroids collide with the planet,
      • ejecting bits of molten rock into the atmosphere
        • that,
          • after cooling and solidifying into glass,
          • (?) back onto Mars's surface.

Understanding the Meaning

The first sentence tells us what scientists discovered:

• 25% of Mars's sand is made of "impact spherules" (spherical bits of glass).

The second sentence explains how these glass bits form:

These spherical bits of glass form when asteroids collide with the planet,

• So when asteroids hit Mars, these glass spheres are created.

The sentence continues:

• 'ejecting bits of molten rock into the atmosphere'
  • When asteroids collide, they throw melted rock up into the air.

Then we get a description of what happens to these bits of molten rock:

• 'that, after cooling and solidifying into glass...'
  • 'that' is referring back to those 'bits of molten rock'
  • After they cool down and turn solid (becoming glass)...

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

• A. to rain
• B. raining
• C. having rained
• D. rain

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

The complete phrase is:

• 'that, after cooling and solidifying into glass, ______ back onto Mars's surface.'

Now let's understand what this is telling us:

• The word 'that' starts a new thought about what happens to the bits of molten rock
  • It's telling us a complete action these bits perform
• 'After cooling and solidifying into glass'
  • This is just setting up the timing - this phrase tells us WHEN the action happens
  • But it's not the main action itself
• The blank + 'back onto Mars's surface'
  • This needs to be the main action - what the bits actually DO
  • They fall back down to Mars's surface

What do we notice about the structure here?

• 'That' introduces a description of what the bits do - it's starting a clause that needs a complete verb
  • Think of it like: 'bits... that [DO SOMETHING]'
• We already have one main verb in the sentence: 'form'
  • 'These spherical bits of glass form when asteroids collide'
• But this 'that' part needs its own verb to be complete
  • It's describing an action the bits take
• The subject doing this action is 'bits of molten rock' (plural)

So we need: A verb that can stand as the main action in this 'that' clause and matches the plural subject 'bits.'

The correct answer is D. rain

This gives us: 'bits of molten rock... that, after cooling and solidifying into glass, rain back onto Mars's surface.'

• 'Rain' works as the main verb showing what the bits do
• It matches the plural subject 'bits'

GRAMMAR CONCEPT APPLIED

Verbs in Relative Clauses Need to Stand Complete

When you use words like "that," "which," or "who" to add information about something (creating what's called a relative clause in grammar terms), you need a verb that can function as the main action of that clause:

Pattern:

• Noun + that/which/who + complete verb + rest of information
• The verb must be in a form that can stand on its own as the main verb
• It must match (agree with) what it's describing

Forms that CAN work as main verbs:

• Present tense: rain, rains, go, goes
• Past tense: rained, went
• Present perfect with helper: have rained, has gone

Forms that CANNOT work as main verbs on their own:

• Infinitive: to rain, to go
• Participles without helpers: raining, going, having rained

In this question:

• "bits of molten rock that... _____ back onto Mars's surface"
• We need a verb that completes the action
• "rain" (base form/present tense) works: "bits... that rain back"
• This matches the plural subject "bits" and stands as a complete verb

Think of it this way: If you can't use the verb form to make a simple sentence like "They ______," it won't work in a relative clause either. "They rain" (tick mark) works. "They to rain" (cross), "They raining" (cross), "They having rained" (cross) don't work as complete sentences.