Doubt on Parallelism

akash singhal wrote: 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.

GMATGuruNY wrote:

akash singhal wrote: 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???