y/(x-p/2)=4/3

y^2=2px

At the same time x=2p and x= 1/8p.

Add a p/2.

The idea is to convert the multiple of vector into the multiple of length, and then into the distance between point A and point B and the directrix.

Because it is a multiple-choice question, it is more concise to set p to 2, and it is easier to calculate, and the result is the same.