In this section you will hear several conversations. Listen to the conversations carefully and then answer the questions that follow. Questions 1 to 3 are based on the following conversation. At the end of the conversation, you will be given 15 seconds to answer the questions. Now listen to the conversation. How does the professor explain "prerequisite"
A．He gives an example and then defines it. B．He explains the root and prefix of the word. C．He gives an example after defining it. D．He shows the woman a list.