5．-Your sister's plan is not quite practical.Why not try to persuade her to give it up?

　　-My sister is very stubborn，so it is________to try to change her mind.

　　A．by chance      B．in vain

　　C．on purpose      D．for sure

　　答案　B

　　解析　句意为：--你妹妹的计划不是很切合实际。为何不劝她放弃？--我妹妹很固执，所以要她改变主意根本就是白费力气。in vain意为"枉费心机，白费力气"。by chance碰巧，凑巧；on purpose故意地；for sure毫无疑问。

　　6．He wanted to pass the exam and stayed up very late，while this ________created further problems.

　　A．in turn       B．in case

　　C．in doubt       D．in short

　　答案　A

　　解析　考查介词短语的辨析。in turn 轮流，反过来，符合句意。in case假使，以防万一；in doubt 不确定，拿不准；in short总之，简言之。

　　7．The woman sold the eggs ________ seven dollars and they were sold ________half price.

　　A．with；at      B．for；at

　　C．at；in       D．by；on

　　答案　B

　　解析　第一空用for"换取"，意为"这位妇女的鸡蛋卖了七美元"；第二空用at，at...price"以……的价格"。故选B。

　　8．He likes his new job，for he is paid ________ the day.

　　A．through       B．during

　　C．by        D．according

　　答案　C

　　解析　by意为"按……计算"。