Unit One Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Two Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Three Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Four Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Five Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Six Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Seven Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Eight Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Nine Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises Unit Ten Part Ⅰ Understanding and Learning Part Ⅱ Key to the Exercises