Question 18 is based on the passage you have just heard.

     Most American college students need to be efficient readers. This is necessary because full-time students probably have to read several hundred pages every week. They don't have time to read a chapter three or four times. (16) They need to extract as much information as possible from the first or second reading.

    An extraordinarily important study skill is knowing how to mark a book. Students mark the main ideas and important details with a pen or pencil, yellow or blue or orange. Some students mark new vocabulary in a different color. Most students write questions or short notes in the margins. Marking a book is a useful skill, but it's important to do it right. First, read a chapter with one pen in your hand and others next to you on the desk. Second, read a whole paragraph before you mark anything. Don't mark too much. Usually you will mark about 10% of a passage. (17) Third, decide on your own system for marking. For example, maybe you will mark main ideas in yellow, important details in blue and new words in orange. Maybe you will put question marks in the margin when you don't understand something and before an exam. Instead, (18) you just need to review your marks and you can save a lot of time.

(18). How should students prepare for an exam according to the speaker?

解析：细节理解题。根据文章最后一句可知，考试前只需要复习做标记的地方，这样可以省很多时间。因此B项（复习做标记的部分）正确。