nums omr filling

do we have to fill it w a pen bcs they haven't mentioned pencils anywhere. thats so stupid bcs who fills the omr w a pen??? what if u realise that u chose the incorrect option when ur rechecking??? how do u fix that??? for my a lvls i wld fill the sheet while i was solving the mcqs n at the end i wld go over everything again n change my answers if i figured smth out after rechecking.