Sentence Correction

Hi,
Can someone pls help me with this question. I cant seem to understand how to arrive at the right answer........

The OA is A

The last two options are wordy, you don't need "It took lees than 35 years” so D and E are out, plus you need the preposition “after”

B and C are also incorrect since you are not specifying a time frame in which an action took place “in less than 35 years” plus B is incorrect (my experience is stay away form the –ing forms”)

That leaves only A for clarity and conciseness

IMO A

less than 35 years ....their descendants had ....

Ignore most of the part in between the sentence. its meant to complicate you.