SC -Idiom and Comma

[Smoondhra]

Source: Master the GMAT 2012 Practice test1- Diagnostic Q27

Scientist and artist Vinci was, and always will be considered by many, as a singular figure among those whose scientific, artistic and other........

A. was, and always will be considered by many, as
B. was, and always will be considered by many as being
C. was, and always will by many, considered
D. was, and always will be considered by many as,
E. was considered by many and always will be by many

OA is D.
I am confused why is A wrong and D is correct. Why is comma before as was wrong and comma after A is right?
Considered as is a wrong idiom then why is D correct?
Please help me to understand this concept.
Thanks in advence.

[pratik.munjal]

D doesn't sound convincing. I guess, the Official Guide is the best bet for preparation in Verbal.

[jnelson0612]

Source: Master the GMAT 2012 Practice test1- Diagnostic Q27

Scientist and artist Vinci was, and always will be considered by many, as a singular figure among those whose scientific, artistic and other........

A. was, and always will be considered by many, as
B. was, and always will be considered by many as being
C. was, and always will by many, considered
D. was, and always will be considered by many as,
E. was considered by many and always will be by many

OA is D.
I am confused why is A wrong and D is correct. Why is comma before as was wrong and comma after A is right?
Considered as is a wrong idiom then why is D correct?
Please help me to understand this concept.
Thanks in advence.

A is not parallel whereas D is.

D says:
Scientist and Artist da Vinci was,
AND (parallel marker)
always will be considered by many as,

Both phrases are setting up to describe da Vinci. He "was" and is considered "as" . . . . .

A breaks this parallelism by putting the comma before "as".