Known as Earth's 'living skin,' biocrusts are thin layers of soil held together by surface-dwelling microorganisms such as fungi, lichens,...
GMAT Standard English Conventions : (Grammar) Questions
Known as Earth's 'living skin,' biocrusts are thin layers of soil held together by surface-dwelling microorganisms such as fungi, lichens, and cyanobacteria. Fortifying soil in arid ecosystems against erosion, ______
Which choice completes the text so that it conforms to the conventions of Standard English?
a recent study's estimate is that these crusts reduce global dust emissions by 60 percent each year.
an estimated 60 percent reduction in global dust emissions each year is due to these crusts, according to a recent study.
these crusts reduce global dust emissions by an estimated 60 percent each year, according to a recent study.
a recent study has estimated that these crusts reduce global dust emissions by 60 percent each year.
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
- Known as Earth's 'living skin,'
- biocrusts are thin layers of soil
- held together by surface-dwelling microorganisms
- such as fungi, lichens, and cyanobacteria.
- held together by surface-dwelling microorganisms
- biocrusts are thin layers of soil
- Fortifying soil in arid ecosystems against erosion,
- (?) [information about these crusts reducing dust emissions by 60% according to a study]
Understanding the Meaning
Let's start reading from the beginning.
The first sentence tells us:
- 'Known as Earth's 'living skin,'
- This is a descriptive phrase - it's what something is called
- 'biocrusts are thin layers of soil held together by surface-dwelling microorganisms such as fungi, lichens, and cyanobacteria'
- So biocrusts are these thin soil layers
- They're held together by tiny organisms like fungi and lichens
- And they've got this nickname: Earth's 'living skin'
Now the second sentence begins:
- 'Fortifying soil in arid ecosystems against erosion,'
- This phrase is describing an action - fortifying (strengthening) soil against erosion
- It's happening in dry ecosystems
This is where we have the blank.
Let's look at our choices:
- A starts with "a recent study's estimate..."
- B starts with "an estimated 60 percent reduction..."
- C starts with "these crusts..."
- D starts with "a recent study..."
Now here's the key question: What is doing the fortifying? What is strengthening the soil against erosion?
The answer is the biocrusts! The crusts are what fortify the soil.
What do we notice about the structure here?
- When a sentence begins with a describing phrase like "Fortifying soil in arid ecosystems against erosion,"
- whatever comes right after the comma must be what's doing that action
- In this case, it must be the crusts - they're the ones fortifying the soil
- If we put something else after the comma (like "a recent study" or "a reduction"),
- it would sound like the study or the reduction is fortifying the soil
- but that doesn't make logical sense - studies and reductions don't fortify soil!
So we need Choice C: "these crusts" as the subject right after the comma.
The complete sentence tells us:
- These biocrusts fortify soil against erosion,
- and by doing so, they reduce global dust emissions by an estimated 60% each year,
- according to a recent study.
GRAMMAR CONCEPT APPLIED
Placing Modifiers: Descriptive Phrases at the Start of Sentences
When a sentence begins with a describing phrase (called a participial phrase in grammar terms) that shows an action, the subject that comes immediately after the comma must be whatever is doing that action.
Pattern: [Describing phrase showing action], [the thing doing that action] + [rest of sentence]
Example 1:
- Wrong: "Running through the park, the trees looked beautiful to Sarah."
- This suggests the trees were running!
- Correct: "Running through the park, Sarah noticed the beautiful trees."
- Sarah is doing the running
Example 2:
- Wrong: "Having studied for weeks, the test was easy for Maria."
- This suggests the test studied for weeks!
- Correct: "Having studied for weeks, Maria found the test easy."
- Maria is the one who studied
In this question:
- The describing phrase is: "Fortifying soil in arid ecosystems against erosion"
- What's doing the fortifying? The biocrusts (these crusts)
- So "these crusts" must come right after the comma
- Choice C does this correctly: "these crusts reduce global dust emissions..."
The key is to always ask: What is logically doing the action described in the opening phrase? That must be your subject right after the comma.
a recent study's estimate is that these crusts reduce global dust emissions by 60 percent each year.
"a recent study's estimate is that these crusts reduce global dust emissions by 60 percent each year."
✗ Incorrect
- This creates a structural problem because it makes "a recent study's estimate" appear to be what's fortifying the soil
- An estimate (a calculation or number) can't physically fortify soil
- The actual subject that does the fortifying (these crusts) ends up buried inside the sentence instead of right where it needs to be
an estimated 60 percent reduction in global dust emissions each year is due to these crusts, according to a recent study.
"an estimated 60 percent reduction in global dust emissions each year is due to these crusts, according to a recent study."
✗ Incorrect
- This makes "an estimated 60 percent reduction" seem like it's fortifying the soil
- A reduction is a result or outcome - it can't perform the action of fortifying
- Again, "these crusts" (what actually does the fortifying) is in the wrong position
these crusts reduce global dust emissions by an estimated 60 percent each year, according to a recent study.
✓ Correct
Correct as explained in the solution above.
a recent study has estimated that these crusts reduce global dust emissions by 60 percent each year.
"a recent study has estimated that these crusts reduce global dust emissions by 60 percent each year."
✗ Incorrect
- This makes "a recent study" appear to be fortifying the soil
- A study is research - it can't physically strengthen soil against erosion
- The crusts (the real actors) are trapped in a subordinate position