21. In ancient China, people used to send smoke _____ to warn that the enemy were coming.    A. signs            B. signals        C. symbols          D. marks答案:   B解析:   这个题考四个名词的比较, 其中:  signs 迹象;  signals 信号;   symbols  象征;   marks  分数. 22. The icy conditions made our travel dangerous. So _____ going by car we took the underground.    A. instead of                         B. in addition to    C. as well as                         D. at the cost of答案:   A解析:   instead of 代替,而不是;     in addition to 除了…之外(还有);    as well as  也(除…之外), 既…又…;    at the cost of  以…为代价. 这道题横线后面是动名词形式,所以选项C和D首先排除. 23. Sometimes my job is boring, but _____ I enjoy it.    A. by contrast                        B. by nature    C. on the whole                       D. on the average答案:   C解析:   by contrast 与…相比,后面要跟with, by contrast with与…相比较;   by nature 本性,生性;    on the whole  总的来说;   on the average 平均. 24. I’ll buy the clock _____ it costs.    A. what         B. whatever           C. where          D. however答案:   B解析:   不管花多少钱,我都买下这个钟. 25. _____ Mr.Wilson is well again, he can travel.    A. Now that      B. So far             C. Though         D. Even if答案:   A解析:   Now that 既然;  So far和现在完成时态连用.