1.The driver was about to start the car__ he found a blind man crossing the street
A.after B.while C.when D.until
答案C; 解析：be about to do when...：一般用于过去：表示刚想做……突然（另一件事发生了）
2.__ many times,they eventually got what they had wanted
A.Having failed B.Failing C.Fail D.To fail
A：解析：强调失败多次,也就是说fail动作发生在got之前,所以用完成式,做状语,表示时间.
3.Buses that have___ safey risks must be repaired immediately
A.unique B.stubborn C.innocent D.potential
D：潜在安全危险
4.The days are popular now ___ it is possible for people to stay at home to make a living
A.when B.that C.where D.which
A：when引导定语从句,先行词是days