Each unit contains a wide range of practice material, and it may not be possible to cover all the work in class in situations where class-time is limited. The teacher should then decide which exercises are most appropriate to the specific needs of his students, and which can be omitted.Similarly, many of the exercises can be given to students to do as homework or self-study work. The writing tasks in Section I at the end of each unit are mmeant to be alternatives (选择余地), from which the student or the teacher chooses the most interesting or most appropriate. Students on intensive courses (精读课程) may, of course, have the time to do more than one of the tasks. The units of the course vary slightly in length and level of difficulty, becoming progressively longer and more difficult through the book. Because of this, and the obvious variation (变化) in classroom situations, it is not possible to specify how much class time would be needed to complete a unit. But, as an approximate (大概的) guide, each unit should provide enough work for three to four lessons, plus a written homework assignment. What can a student do with writing tasks He may choose to do the most interesting or_____________________task.