Click on the letter next to the answer you think is correct. You'll see the feedback here in this frame. If you get it wrong, you can try again, till you find the correct answer. Your current score will always be displayed in percentage (number of all answers divided by correct answers). It means that if, for example, you find the answer to the first question only after the fourth try, your score will be 25% only! You don't have to answer all the questions in the given order; you'll always see which questions you have still left.