Question 45: ________ Paul realizes that he was on the wrong flight.

Structure:

No sooner + had + S + PP…+ than + a clause

Eg: No sooner had I arrived home than she left.

Only after + clause 1 + inversion clause (auxiliary verb + S + V…): Only after ….then…

It was not until + time cluster/clause 1 + that + clause 2 divide then QK (Until…then…)

~ Not until + time phrase/ clause 1 + inversion clause (auxiliary verb + S + V…)

Eg: It was not until 2012 that she became a doctor. – Not until 2012 did she become a doctor.

=> Answer D (It was not until the plane took off that Paul realized that he had boarded the wrong flight.)