(1) Now a lot of families own cars and this _________ (cause) serious air pollution.

(2) In Yixing, teapots(茶壶) _________ (produce) in large numbers.

(3) The twins _________ (dig) in the garden when their mother came.

(4) —Your city looks beautiful!

—Yes. Lots of trees _________ (plant) last year.

(5) He is too busy ________ (do) his homework to go fishing with his parents at weekends.

(6) Lucy is wondering where _________ (spend) her summer holiday.

(7) We hope more and more people _____(join) us to protect our environment.

(8) Great changes __________ (take) place in Wuxi in the past five years.