A study has found that certain types of bacteria called hypermutators

OFFICIAL EXPLANATION



OVERVIEW

• USING WORDS OF COMPARISON

If two unequal or dissimilar things are being compared, you are likely to see comparison words such as greater, more, less, smaller, bigger, fewer, and higher.
→ Without exception in this context, you need THAN. You never use AS with these words.
X is greater than Y.
ABC contains more candy than PQR.
Big Bank's interest rate is higher than that of Small Bank.

If two equal or similar things are being compared, and you see one as, look for another as. The word as is never paired with than.
→ Without exception in this context, you need AS. . .AS. In a comparison, you never use than with as.
I am as tall as he.
Oak tree shoots grow as quickly as weeds do.

• MORE v GREATER
→ More can be used to talk about countable nouns.
She bought more apples than she needed. (Apples are countable.)

→ More can be used with many uncountable nouns.
He asked for more water. (Water is not countable.)
A suit made by hand costs more than a suit off the rack.

→ If the noun is itself a number (a measurement, a rate, percent), use greater, not more.
The volume of the glass aquarium is greater than the volume of the plastic aquarium.

Population growth was greater in the early twentieth century than in the later decades of the twentieth century.

This question mentions a rate.
Some bacteria mutate faster than others do.
In this context, "fitness" means "the rate at which bacteria mutate." That is, in this context, fitness is a rate.
A rate is itself a number and thus is modified by greater, not more.

THE PROMPT

The sentence is comparing the rates of mutation of two different kinds of bacteria.
Hypermutators mutate at a faster rate than regular bacteria mutate.
Hypermutators are thus better fit to survive—in other words, the fitness (mutation rate) of hypermutators is greater than the fitness of "regular" bacteria.

THE OPTIONS

• I do not see any errors
→ greater relative fitness correctly refers to the difference between rate at which the hypermutator bacteria mutate and the rate at which regular bacteria mutate
→ greater is correctly paired with than

• We need another AS.
Correct: AS . . . AS
Wrong: AS . . . than
ELIMINATE B


• We need greater relative fitness.
→ Relative fitness refers to the relative rates of mutation of regular and hypermutator bacteria.
A rate is itself a number.
Nouns that themselves are numbers take greater, not more. (See the Overview.)
We modify a rate with greater, not more.

• that mutate (in options A, B, C, and D) is stronger than mutating because the former highlights a defining feature of a type of bacteria.
→ often, there is little difference in meaning between a relative clause (that mutate) and a participle (mutating).
In fact, we can often "reduce" relative clauses to a participle.
In this case, the two are not quite interchangeable and that mutate is better than mutating.
→ This sentence centers on the difference between essential characteristics of hypermutator bacteria and regular bacteria.
If we read . . . than bacteria mutating at a slower rate, we do not know whether this slower rate of mutation is an essential characteristic of the bacteria or just something that the bacteria are doing at the time.
Yes, it is fairly clear from context that this slower rate of mutation is an essential characteristic of the bacteria that are not hypermutators.
But using that mutate tells us for certain that these "regular" bacteria always mutate at a slower rate than that of hypermutator bacteria.

I would use this face-off in a tiebreaker or if I were not sure about more/greater.
Optimally, we would eliminate Option C because more should be greater.
ELIMINATE C

• We need greater relative fitness; more relative fitness is wrong.
See the explanation beneath option C.

• IF "more" were correct (and it is not), we would need a THAN.
Correct: More . . . than
Wrong: More . . . as
ELIMINATE D

• We need a THAN.
Correct: Greater . . . than
Wrong: Greater . . . as
ELIMINATE E

Notes -- recap
Greater . . . THAN
More . . . THAN
AS . . . AS

Use "greater than" for nouns that are themselves numbers (distance, area, rate, percent).

The answer is A.

COMMENTS

keyuwang , welcome to SC Butler.

This question is slippery.
If you learn the first three lines of "Notes," above, you can eliminate three answers: B, D, and E.

Another good lesson from this question is that the sentence itself tells us that fitness is a rate.
Even if you cannot remember the rule, try to think of a sentence that might help:
A train travels at more speed than a horse does.
A train travels at greater speed than a horse does.

As far as I can tell from time stamps, everyone posted before the OA was revealed.
Correct answers with explanations get kudos.

Sumi1010 , I am bumping you to Best Community Reply.

Stay safe, everyone.