When you don't know the answer to a question, it's best to leave it blank and
come back to it later because:
A. you might get clues to the answer as you go through the rest of
the exam.
B. you should only write down things that you're absolutely certain
are correct
C. you can't be marked wrong for questions you don't attempt.
D. you won't get partial credit if you put down things that aren't true.