If certain atmospheric particles _____ Earth from distant supernovae and carry specific isotopic signatures—a possibility that astrophysicists continu...

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

• If certain atmospheric particles [?] Earth
  • from distant supernovae
  • and carry specific isotopic signatures
    • -a possibility
      • that astrophysicists continue to investigate-
• the particles offer crucial evidence
  • about the chemical evolution of our solar system.

Understanding the Meaning

Let's start reading:

'If certain atmospheric particles _____ Earth from distant supernovae...'

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

• reach
• had reached
• will reach
• are reaching

These are all different tenses of the same verb. To see what fits, let's read the rest of the sentence and understand what it's saying!

The sentence continues:

• 'and carry specific isotopic signatures'
  • So the particles do two things: they [blank] Earth AND they carry signatures
• '-a possibility that astrophysicists continue to investigate-'
  • This is just a side note telling us scientists are still studying whether this actually happens
• 'the particles offer crucial evidence about the chemical evolution of our solar system.'
  • This is the result - IF the particles do reach Earth and carry these signatures, THEN they offer evidence

So the complete meaning is:

• IF these particles from distant supernovae reach Earth and carry specific signatures,
• THEN they would give us important evidence about how our solar system developed chemically

What do we notice about the structure here?

• This is a conditional sentence - an "If...then" statement
  • The "if" part describes the condition: particles doing two things
  • The "then" part describes the result: offering evidence
• Within that "if" part, we have two actions connected by "and":
  • particles [blank] Earth
  • particles carry signatures
  • These two verbs need to match in form - they're doing parallel jobs in the sentence
• Look at "carry" - it's present tense, simple form
  • Not "carried" or "will carry" or "are carrying"
  • Just "carry"
• Also notice: "astrophysicists continue to investigate"
  • This tells us we're talking about a current, ongoing possibility
  • In English, when we express these kinds of hypothetical possibilities in "if" clauses, we use present tense

So we need reach - present tense, simple form - to match "carry" and to fit the conditional sentence pattern.

The correct answer is A: reach

GRAMMAR CONCEPT APPLIED

Parallel Structure with Coordinating Conjunctions and Conditional Sentence Patterns

When two or more verbs are connected by "and" (or "or"/"but") within the same clause, they need to match in form-this is called parallel structure in grammar terms:

Pattern with coordinating conjunctions:

• ✓ The company will expand operations and will hire new staff
• ✓ The company will expand operations and hire new staff (both using base form with shared "will")
• ✗ The company will expand operations and hired new staff (mixing future and past)

In our question:

• The particles reach Earth and carry signatures
• Both verbs are present simple
• They work in parallel, connected by "and"
• They must match in form

Additionally, conditional sentences follow specific tense patterns:

When expressing hypothetical possibilities or general truths, we use present tense in the "if" clause:

• ✓ If the particles reach Earth, they offer evidence
• ✗ If the particles will reach Earth, they offer evidence

This is true even when we're talking about future or hypothetical events. The "if" clause uses present tense to express the condition.