动词填空 用括号内所给动词的适当形式填空。(本大题共8分，每小题1分)
小题1:Now a lot of families own cars and this _________ (cause) serious air pollution.
小题2:In Yixing, teapots(茶壶) _________ (produce) in large numbers.
小题3:The twins _________ (dig) in the garden when their mother came.
小题4:—Your city looks beautiful!
—Yes. Lots of trees _________ (plant) last year.
小题5:He is too busy ________ (do) his homework to go fishing with his parents at weekends.
小题6:Lucy is wondering where _________ (spend) her summer holiday.
小题7:We hope more and more people _________ (join) us to protect our environment.
小题8:Great changes __________ (take) place in Wuxi in the past five years.