The alga species Chlorella vulgaris is very efficient at making oxygen. For this reason, scientists are currently exploring ways to...

Sentence Structure

• Sentence 1:
  • The alga species Chlorella vulgaris
    • is very efficient at making oxygen.
• Sentence 2:
  • For this reason,
    • scientists
      • are currently exploring ways to use this species in space.
• Sentence 3:
  • C. vulgaris might be used,
    • for example,
    • to build future biological air exchange systems
      • that (?) oxygen for astronauts.
• Blank: (?) = are producing / produced / produce / have produced

Understanding the Meaning

Let's read through and understand what's being said:

The first sentence tells us:

• The alga species Chlorella vulgaris is very efficient at making oxygen.
  • So we have an algae that's really good at producing oxygen.

The second sentence:

• For this reason, scientists are currently exploring ways to use this species in space.
  • Because it makes oxygen so well, scientists want to figure out how to use it in space.

Now the third sentence gives us an example:

• C. vulgaris might be used, for example, to build future biological air exchange systems...

Now here's where we need to fill in the blank:

• "systems that ______ oxygen for astronauts"

Let's look at our choices:

• The choices give us different forms of a verb: are producing, produced, produce, have produced
• What varies is the tense and form

What do we know?

• The verb's subject is "systems" - from "biological air exchange systems that ______"
  • "that" refers back to "systems"
  • "systems" is plural, so we need a plural verb
• These are "future biological air exchange systems"
  • They don't exist yet
  • The sentence is describing what these systems would do - their function
• When we describe what something does as its basic function or purpose, we use simple present tense, even when talking about future things.
  • Like: "We'll build a machine that processes data" (not "will process" or "is processing")

So we need:

• Simple present tense (to describe the function)
• Plural form (to match "systems")
• That's "produce"

The answer is C: "systems that produce oxygen for astronauts"

GRAMMAR CONCEPT APPLIED

Using Simple Present Tense to Describe Functions and Characteristics

When you describe what something does as its basic function or characteristic - even if you're talking about something in the future - you use simple present tense. The verb must agree in number with what it's describing.

The pattern:

• [Noun] + that + [simple present verb matching in number]

Examples:

1. Future context, but describing function:
   • "Next year we'll install a system that filters water"
   • "filters" is simple present (even though the system doesn't exist yet)
   • We're describing what the system does - its function
2. With plural noun:
   • "They're designing robots that perform surgery"
   • "perform" is simple present plural
   • Matches the plural "robots"

In our question:

• "future biological air exchange systems that produce oxygen"
• "produce" is simple present plural
• Matches "systems" (plural)
• Describes the function of these systems
• Works even though we're talking about future systems

This is different from describing a specific action happening at a specific time, where you would need other tenses (past, future, progressive, etc.).