The Right Answer grading option for the Multi-Select quiz question type is problematic. Students not only receive credit for selecting right answers, but they also receive credit for not selecting wrong answers. If your Multi-Select question has two correct answers and two wrong ones, the student who chooses one correct answer and nothing else receives credit for selecting the correct answer plus credit for not selecting the two wrong answers, therefore earning 3/4 of the points allotted to that question. Unfortunately, the only time students won't get credit for not selecting wrong answers is if they don't select anything at all!