Why is A not the answer? Why D and why not A?

[dgr8sandeep]

I saw this question in another thread but wasnt able to post my question there, hence asking it here.

In 1860, the Philological Society launched its effort to create a dictionary more comprehensive than the world had ever seen; although the project would take more than 60 years to complete, the Oxford English Dictionary had been born.
A)would take more than 60 years to complete, the Oxford English Dictionary had been
B) took more than 60 years to complete, the Oxford English Dictionary was
C) would take more than 60 years to complete, the Oxford English Dictionary was being
D) would take more than 60 years to complete, the Oxford English Dictionary was
E) took more than 60 years to complete, the Oxford English Dictionary was about to be

What is the difference between had been and was here?

Why is A not the answer? Why D and why not A?

[RonPurewal]

per the forum rules, we need a citation of the original source of this problem. please provide one; thank you.