Like other amphibians, the wood frog (Rana sylvatica) is unable to generate its own heat, so during periods of subfreezing...

Sentence Structure

• Like other amphibians,
  • the wood frog (Rana sylvatica)
    • is unable to generate its own heat,
• so during periods of subfreezing temperatures,
  • it (?)
    • by producing large amounts of glucose,
      • a sugar that helps prevent damaging ice
        • from forming inside its cells.
• Where (?) = had survived / survived / would survive / survives

Understanding the Meaning

Let's start from the beginning:

• 'Like other amphibians, the wood frog (Rana sylvatica) is unable to generate its own heat'
  • This tells us a basic biological fact about wood frogs
  • They can't produce their own body heat
  • Notice the verb here: 'is unable' - present tense
  • This is describing a general characteristic, not a one-time event

The sentence continues:

• 'so during periods of subfreezing temperatures, it ______'

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

• had survived (past perfect)
• survived (simple past)
• would survive (conditional/hypothetical)
• survives (simple present)

So we're deciding which tense of 'survive' fits. To see what works here, let's read the rest of the sentence and understand what it's saying!

• 'by producing large amounts of glucose, a sugar that helps prevent damaging ice from forming inside its cells'
  • This explains HOW the frog survives the cold
  • It makes a type of sugar that protects its cells from ice damage

What do we notice about the whole sentence?

• The entire sentence is describing a general biological process
  • Not a specific event that happened once in the past
  • Not a hypothetical "what if" scenario
  • But a repeated, ongoing fact about what wood frogs do
• The opening establishes this with 'is unable' (present tense)
  • This signals we're talking about general characteristics
  • A timeless fact about the species
• The blank describes what happens when cold temperatures occur
  • This happens repeatedly - every time it gets cold
  • It's part of the frog's regular biological response

For general truths and repeated actions - facts that are always true - we use simple present tense.

So we need: survives (Choice D)

GRAMMAR CONCEPT APPLIED

Using Simple Present Tense for General Truths and Facts

When describing scientific facts, general truths, or habitual actions that happen repeatedly (rather than specific one-time events), we use simple present tense. This is especially important in scientific writing:

General biological fact:

• "The wood frog is unable to generate its own heat"
• Not "was unable" - because it's always true

Habitual action that repeats:

• "During cold periods, it survives by producing glucose"
• Not "survived" or "would survive" - because this happens every time

Why this matters in this question:

The sentence begins with "is unable" (present tense), establishing that we're discussing general characteristics of wood frogs. Everything that follows must maintain this timeframe. The verb "survives" continues describing what the frog does as a regular biological process - not a one-time past event or hypothetical scenario, but a repeated, ongoing fact about the species.

Think of it like this: If you're writing "Water freezes at 32°F," you wouldn't say "Water froze at 32°F" or "Water would freeze at 32°F" - because you're stating a fact that's always true. The same principle applies here with the wood frog's survival mechanism.