In GRE General Test, the verbal and quantitative reasoning sections are Computer Adaptive Tests (CAT). The Test is given on a computer system and the software adjust questions difficulty level according to your performance. That means difficulty level of one question is based on your performance( correctness of choice made, time taken etc..) on the previous question.

At the start of the test, you are presented with test questions of middle difficulty. As you answer each question, the computer scores it and uses that information, as well as your responses to preceding questions and information about the test design, to determine the next question. As long as you answer correctly, you will typically be given questions of increased difficulty. When you respond incorrectly, you will typically be given less difficult questions.

Because the computer scores each question before selecting the next one, you must answer each question when it is presented. For this reason, once you answer a question and move on to another, you cannot go back and change your answer. The computer has already incorporated both your answer and the requirements of the test design into its selection of your next question.