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

OA: D

Please explain..