Doubt on Parallelism

[akash singhal]

Each year companies in the United States could save as much as $58 billion annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided offices with cleaner air.

(A) annually by preventing illness among employees and gain as much as $200 billion through improving
performance of workers if they simply provided
(B) annually if they prevented employee illness and gain as much as $200 billion through worker performance improved by simply providing
(C) annually in employee illness prevention and gain as much as $200 billion through worker performance improved by simply providing
(D) in employee illness prevention and gain as much as $200 billion through improving performance of workers if they simply provided
(E) by preventing illness among employees and gain as much as $200 billion through improved worker performance if they simply provided

OE E

Well I can easily eliminate A,B and C for the use of redundant word anually.
Please explain in a bit detail why E and Not D.
Idiom problem or adjective issue?or both?

[GMATGuruNY]

Please explain in a bit detail why E and Not D.
Idiom problem or adjective issue?or both?

Errors in D:

could save as much as $58 billion in employee illness prevention
Here, the conveyed meaning is that a PARTICULAR COST -- the cost of illness prevention -- could be reduced by $58 billion.
The intended meaning of the original sentence is that TOTAL COSTS could be reduced by $58 billion and that preventing illness is HOW these total costs could be reduced.
Since D does not convey the intended meaning, eliminate D.

through improving performance of workers
Interpretation 1: COMPANIES will be IMPROVING the performance of workers.
Interpretation 2: The PERFORMANCE of workers will be IMPROVING on its own.
Since the intended meaning is not crystal clear, eliminate D.

improving performance of workers if they simply provided offices
Here, a reader might construe that they serves to refer to workers.
Since the intended referent for they is companies, eliminate D.

[akash singhal]

Please explain in a bit detail why E and Not D.
Idiom problem or adjective issue?or both?

Errors in D:

could save as much as $58 billion in employee illness prevention
Here, the conveyed meaning is that a PARTICULAR COST -- the cost of illness prevention -- could be reduced by $58 billion.
The intended meaning of the original sentence is that TOTAL COSTS could be reduced by $58 billion and that preventing illness is HOW these total costs could be reduced.
Since D does not convey the intended meaning, eliminate D.

through improving performance of workers
Interpretation 1: COMPANIES will be IMPROVING the performance of workers.
Interpretation 2: The PERFORMANCE of workers will be IMPROVING on its own.
Since the intended meaning is not crystal clear, eliminate D.

improving performance of workers if they simply provided offices
Here, a reader might construe that they serves to refer to workers.
Since the intended referent for they is companies, eliminate D.

Firstly, as usual Mitch you are the best in explaining every tiny detail.Thanks for that.

Next, in D and E the use of the word "improving" and "improved", Is there any fact on when one should opt for past participle and when opt for present participle???

[GMATGuruNY]

Firstly, as usual Mitch you are the best in explaining every tiny detail.Thanks for that.

Next, in D and E the use of the word "improving" and "improved", Is there any fact on when one should opt for past participle and when opt for present participle???

The following construction will allow for more than one interpretation -- as discussed in my post above -- and thus is not viable:
through + VERBing + NOUN.