Why would you get a decrease in marks?!?! If you got the answers right...they dont care about the other answers However do not right something that is right and something wrong in the same sentence. Do it it different sentences.
Hmmm I'm not sure if you get a mark taken off; I know you do in some subjects. Look at a mark scheme for the past paper and see what it says under notes
Positive marking only really applys heavily to essay questions like if you get a date confused with an event in History they wont mark you down they just wont give you a mark for remembering a date.
In Q&A exams like the one above they will do pretty much the same. If you awsner 2 points correctly and 1 wrong they will mark you for the 2 you got right.