If simple sugars such as ribose and glycolaldehyde ______ Earth from elsewhere and survived impact—a possibility astrophysicist Nicolle Zellner outlin...

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 simple sugars
  • such as ribose and glycolaldehyde
  • [?] Earth from elsewhere
  • and survived impact
    • —a possibility
      • astrophysicist Nicolle Zellner outlined in a 2020 study—
• the sugars could have reacted with other molecules
  • that were already present on the planet
  • to form the nucleotides
    • that are the structural components of RNA and DNA.

Understanding the Meaning

The sentence starts with a conditional scenario:

• "If simple sugars such as ribose and glycolaldehyde..."
  • We're talking about a hypothetical situation involving these specific sugar molecules.

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

1. reach
2. had reached
3. will reach
4. are reaching

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

The IF part continues:

• "[blank] Earth from elsewhere and survived impact"
  • This is describing two things that would happen: the sugars arriving on Earth from space AND surviving the impact of landing.
  • Notice "survived" is already in past form here.
• "—a possibility astrophysicist Nicolle Zellner outlined in a 2020 study—"
  • This is just adding context - telling us this scenario is something a scientist has considered.

Now the main part of the sentence (the "then" part of the IF-THEN):

• "the sugars could have reacted with other molecules that were already present on the planet"
  • This tells us what WOULD have happened if those sugars had arrived.
  • "Could have reacted" is key here - this is talking about a hypothetical result in the past.
• "to form the nucleotides that are the structural components of RNA and DNA"
  • This explains why this would matter - these reactions could have created the building blocks of genetic material.

What do we notice about the structure here?

• This is a past conditional sentence - discussing a hypothetical scenario about the past:
  • IF clause: "If sugars [?] Earth and survived"
  • THEN clause: "the sugars could have reacted"
• The main clause uses "could have reacted"
  • This form (modal + have + past participle) signals we're in past conditional territory.
  • This tells us something about what the IF clause needs.
• For past conditionals, there's a matching pattern:
  • IF clause needs: had + past participle
  • THEN clause has: could/would/might + have + past participle
• Here's what happens with "had reached and survived":
  • The "had" applies to both verbs
  • It's like saying "had [reached Earth and survived impact]"
  • Both "reached" and "survived" work as past participles after the shared "had"

So we need B. had reached to complete the past conditional structure that matches with "could have reacted" in the main clause.

GRAMMAR CONCEPT APPLIED

Past Conditional Sentences (Hypothetical Past Situations)

When discussing hypothetical scenarios about the past - things that didn't actually happen but we're imagining "what if they had" - English uses a specific conditional pattern (called the third conditional in grammar terms):

IF clause: had + past participle
Main clause: modal verb (could/would/might) + have + past participle

Example 1:

• "If the meteorite had contained water, it could have helped form Earth's oceans."
  • IF part: had contained (past perfect)
  • THEN part: could have helped (past conditional)

Example 2:

• "If ancient bacteria had survived in the ice, scientists would have discovered them."
  • IF part: had survived
  • THEN part: would have discovered

Example 3 (compound verbs in IF clause):

• "If the expedition had reached the summit and planted the flag, history would have remembered them."
  • The "had" applies to both "reached" and "planted"
  • Both work as past participles sharing the auxiliary "had"

In this question:

• IF clause needs: "had reached... and survived" (past perfect for both verbs)
• Main clause has: "could have reacted" (past conditional)
• This creates the proper conditional structure for discussing what might have happened in Earth's early history