Monthly pay is different every month - which category

[Zahir27]

Hi
I have two questions, im applying for a spouse visa so that my wife can join me in the UK from Pakistan.
I get paid £19.5K a year but I also get bonus and commision on top. My average for last tax year was £37.2K and last six months was £37.4K
Some months pay is higher than the other.. which category would I apply for A, or B? Even the lowest month still sees me above the threshold they are looking for.

Final question, I filled out application form online everything is correct on it. Paid the fee online etc and booked appointment for biometric.
On the appointment letter which my wife needs to take to Lahore for biometric has her old passport number on it.
This must be a glitch or system error. i dont know. Application form shows her new passport and old passport nunber under any old passport held. Would this cause a problem? Any ideas? Would really appreciate your help!
Thanks