"If you are not sure of a question, select you best answer and move on. Avoid Guessing"
Now, it's a bit contradictory... since obviously if you are not sure, then your 'best' answer will be a guess.
I think that when they say "If you are not sure", they dont mean "if you have no idea, have no time left and absolutely want to give an answer". Plus finding out random answers is obvious: starting from a certain point you will have mostly wrong answers.
Anyway, the purpose of the test is precisely to make you say when you cant give an answer to a question...