If the same rules apply to the SAT as they do to the ACT it would have been best to leave them blank if you didn't know them instead of guessing. As not answering would mean you wouldn't get any points, yet also won't get points off. The only time you should guess is when you've narrowed the choices down to at least three or two. That is for the SAT though, I'm not sure if the ACT is similar in that aspect.