Atoms in a synchrotron, a type of circular particle accelerator, travel faster and faster until they ________ a desired energy...

Sentence Structure

• Atoms in a synchrotron,
• a type of circular particle accelerator,
• travel faster and faster
• until they [?] a desired energy level,
• at which point they are diverted to collide with a target,
• smashing the atoms.
• Where [?] = will reach / reach / had reached / are reaching

Understanding the Meaning

Let's start from the beginning:

'Atoms in a synchrotron, a type of circular particle accelerator, travel faster and faster'

• We're talking about atoms inside a synchrotron (which the sentence helpfully tells us is a circular particle accelerator)
• These atoms travel faster and faster - they're accelerating

This is where we have the blank: 'until they ______ a desired energy level'

• Let's look at the choices - they're all forms of "reach" but with different tenses:
  • will reach (future)
  • reach (simple present)
  • had reached (past perfect)
  • are reaching (present progressive)

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

'at which point they are diverted to collide with a target, smashing the atoms'

• Once the atoms reach that desired energy level, they get redirected
• They collide with a target, which breaks apart the atoms

So the complete picture is:

• This sentence is describing how a synchrotron works - the general process
• Atoms travel faster → reach an energy level → get diverted → collide with a target

What do we notice about the verb forms here?

• 'Travel' is in simple present tense
  • This tells us we're describing a general process or how things typically work, not a specific event in the past or future
• 'Are diverted' is also in simple present
  • Again, describing the general process
• All the verbs are describing a repeating, general process

Now, here's an important pattern: when we use time words like "until," "when," "before," or "after" to describe a general process, we use simple present tense - even though the action happens in the "future" relative to the earlier action.

• We say: "They travel until they reach" (not "until they will reach")
• We say: "When water reaches 100°C, it boils" (not "when water will reach")

So we need: reach (choice B)

This keeps all the verbs in simple present, maintaining consistency while describing the general process of how synchrotrons work.

GRAMMAR CONCEPT APPLIED

Using Simple Present in Time Clauses for General Processes

When you describe a general process or habitual action with time words like "until," "when," "before," "after," or "as soon as," you use simple present tense - even when the meaning involves future-relative actions. You don't use "will."

Pattern examples:

• General process: Water boils when it reaches 100°C.
  • NOT: "when it will reach"
  • The simple present "reaches" is correct in the time clause
• Habitual action: I always save my work before I close the program.
  • NOT: "before I will close"
  • The simple present "close" is correct

In this question:

• "Atoms travel faster and faster until they reach a desired energy level"
  • "Travel" = simple present (general process)
  • "Reach" = simple present (correct in the "until" time clause)
  • "Are diverted" = simple present (continuing the general process)

All verbs stay in simple present to consistently describe how synchrotrons generally work.

Note: This rule specifically applies to time clauses (those introduced by until, when, before, after, as soon as, etc.). In other contexts, we can use future tense normally. For example, "They will reach the target tomorrow" is fine because it's not a time clause.