Problem with a Question!

[uz]

Hi Erin I'm having problem with this Question, please help me

Not until geologist [began] to study [exposed] rocks in ravines and [on] mountainsides [they did] discover many of the earth's secrets.


As per book correct answer is "D". Shouldnt it be 'did begin' ,means "A" should be correct.

[vehemence]

Hi, uz!
I think the correct answer is D. I always remember an example when I have question like this.The example I always remember is
- Not until she had apologized, did I speak to her.

[yellowredandwhite]

I agree

[Idiot]

D is correct answer. There is a conversion structure here