The basic checking is like this - Mark correct answer on your big sheet:
Question 45,736:
ABCDEF
a)ABDFCE b) BACDEF c) ABCDEF d) AFCDEB e) ABCDEG
I think you can only mark one answer as these things are all marked by computer me thinks and it would get all confused by two although don't take my word for it, they will tell you in instructions they don't try to deliberately trick you too much.
O Yer and fyi I think they go up to 8 digits - fun and games.
Letters is easier as you can normally find a funny little word in the combinations they use and then find that in the options, numbers is just plain annoying .
Goodluckage
Rich