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
Explanation from answer key: The past perfect ("had been born") is used when there are two past actions and we want to indicate which one happened first. In the underlined portion of the sentence, however, the other verb, "would take," is not in the past tense, so we need to use the simple past "was born." (Remember that we always use the most simple tenses allowed; the perfect tenses, and other complicated tenses, are used only when required by the sentence structure.) The second half of the sentence stands in contrast to the first half, in which the simple past "launched" is correctly paired with the past perfect "had seen."
(D) CORRECT. This choice correctly uses the simple past "was born." A more complicated past tense is not required because the other verb "would take," is not in the past tense .
I'm confused about the explanation in (D). I thought "would" is a past tense? I chose (B).