Multiple choice, multiple answer with penalty

Currently, each correct answer adds points and each wrong answer discount points. Thus, if you create a new question with three possible answers but only two of them being correct, if the students marks the three he/she will get 2/3 of the total points when, probably, he/she should be assigned zero points. Then our instructors have requested us to choose between:

• If you need to mark all correct answer to get points in the question, otherwise you'll get a zero or the discount set up by the instructor.
• Each correct answer gives you points but wrong answer has a porportional penalty, i.e, if the whole penatly is 0.5 and the student marks only 2/3 correct answers, then he/she will get 0.5 * 1/3 = 0.16 as discount.