After having had a few beers and created a rather nice looking spreadsheet to work this out, resplendant with nice colours - I've concluded I'm wasting my time.

Logic has struck and I'll go with answer 'A' as well.

It works on the principle of the 'random seated' pax 'overflowing.

The first 99 pax to board have, obviously, 99 seats allocated to them - of which 98 will be taken (albeit they might not actually be taken by the person who should. That leaves 2 seats - 1 of which will be yours.