Cars play an important role in modern life, but they_______also cause many problems like air pollution．

Jack is late again. It isofhim to keep others waiting.

He was hiding behind the door _________ hecould see what was happening

My uncle is fixing up some old bikes these days. He plans to to the children in poor villages.

After Mandela was free (自由的) in 1990, he chose to shake hands with the people ________ wanted to kill him.