There is no deduction for a wrong answer on the multiple choice section, so a blank is a guaranteed zero while a guess carries positive expected value. With four options a blind guess is worth a quarter of a mark, eliminating one lifts it to a third, and eliminating two makes it a coin flip. Fill every answer. On the free response side the equivalent rule is to write something for every part, because rubrics award points for correct steps even when the final answer is wrong, and an empty part can earn nothing at all.