Sentence Correction

Overuse of antibiotics often leads to a reduction in the effectiveness of the drugs; this theory has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

A Overuse of antibiotics often leads to a reduction in the effectiveness of the drugs; this theory has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

B If overused, one theory in regard to antibiotics becoming reduced in their effectiveness has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

C If overused, one theory regarding antibiotics becoming reduced in their effectiveness has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than those who rarely take antibiotics.

D The observation that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics has recently given support to one theory in regard to this phenomenon, if overused, antibiotics become reduced in their effectiveness.

E The observation of people who regularly take antibiotics having fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics has recently bolstered one theory about why antibiotics become reduced in their effectiveness.

OA is A
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Roland2rule wrote:Overuse of antibiotics often leads to a reduction in the effectiveness of the drugs; this theory has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

A Overuse of antibiotics often leads to a reduction in the effectiveness of the drugs; this theory has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

B If overused, one theory in regard to antibiotics becoming reduced in their effectiveness has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics.

C If overused, one theory regarding antibiotics becoming reduced in their effectiveness has recently been bolstered by observations that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than those who rarely take antibiotics.

D The observation that people who regularly take antibiotics have fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics has recently given support to one theory in regard to this phenomenon, if overused, antibiotics become reduced in their effectiveness.

E The observation of people who regularly take antibiotics having fewer disease-fighting organisms in their bodies than do people who rarely take antibiotics has recently bolstered one theory about why antibiotics become reduced in their effectiveness.

B and C: If overused, one theory
Here, overused seems to refer to theory, implying that ONE THEORY might be OVERUSED.
This meaning is nonsensical.
The intended meaning of the original sentence is that ANTIBIOTICS might be overused.
Eliminate B and C.

D: The observation...has recently given support to one theory in regard to this phenomenon, if overused, antibiotics become reduced in their effectiveness.
Here, each colored portion constitutes a complete sentence.
As a result, the comma between the two colored portions can be replaced by a PERIOD.
If a comma can be replaced by a period -- an error known as a COMMA SPICE -- the answer choice is WRONG.
Eliminate D.

E: people...having fewer disease-fighting organisms than do people who rarely take antibiotics
Here, do seems to refer to having (an adjective modifying the people in blue).
Question: What KIND of people?
Answer: people HAVING fewer disease fighting organisms.
The referent for do must be a preceding VERB; do cannot stand in for an adjective.
Eliminate E.

The correct answer is A.