Unit 15 At home with the twins 1
Unit 16 What's it made of？ 11
Unit 17 What was it used for？ 20
Unit 18 Planting trees 31
Unit 19 Mainly revision 44
Unit 20 The world's population 54
Unit 21 Shopping 63
Unit 22 At the doctor's 72
Unit 23 The football match 81
Unit 24 Mainly revision 92
参考答案 101