20．听短文，根据短文内容在各题所给的 A、B、C 三个选项中，选出一个最佳选项．（本题听两遍）
21．Mr．Smith is explaining the importance of in the science lesson
A．air B．food C．water
22．Why didn't the students want to answer the teacher？
A．Because they think the question is too easy．
B．Because they don't know how to answer the question．
C．Because they want to answer the question after the lesson．
23．What does the water in the river behind the boy's house look like？
A．Black B．Red C．Green
24．Why can't people swim in that river now？
A．Because there are dangerous animals in the river．
B．Because the river is too dirty．
C．Because the water is too deep．
25．What did Mr．Smith want to do？
A．He wants to swim in the river．
B．He wants to go fishing in the river．
C．He wants to make water clean．
