Mike：Our city is dirty now．
Helen：Yes．Why does our city become dirty？
Liu Tao：There is so much smoke from cars and the factories．
Mike：I think so．Rubbish makes our city dirty too．
Helen：We should protect our city．
Liu Tao：We can take the bus and the metro to school．We can plant more trees．
Helen：We can also save water and save trees to protect our Earth．
Mike：Don't use too much plastic．

根据对话内容，将句子补充完整．
55．Our city is

dirty

dirty

now，because there is so much

smoke

smoke

from cars and factories．
56．We can

take the bus and the metro

take the bus and the metro

to school to protect our city．
57．We can also

save water and save trees

save water and save trees

to protect our Earth．
58．What else can we do（我们还能做什么） to protect our Earth？

Don't use too much plastic．

Don't use too much plastic．