19. We live day by day, but in the great things, the time of days and weeks ______ so small that a day is unimportant.

A. is                         B. are                       C. has been               D. have been

分析：A。句子主語為the time，故用謂語要用單數; 再根據句子前后的時態live和is，可知此處宜用一般現在時，不用現在完成時，故選A。

20. — ______ you ______ him around the museum yet?

— Yes. We had a great time there.

A. Have; shown         B. Do; show              C. Had; shown          D. Did; show

分析：A。根據問句中的yet可知，所談論的事情發生在最近的過去，且問話者很關心這個過去發生的事情對現在的影響和結果，所以用現在完成時。注意：不要受答語中過去式謂語had的影響而誤選C。

21. —I have got a headache.

—No wonder. You ______ in front of that computer too long.

A. work                    B. are working          C. have been working    D. worked

分析：C。由于在電腦前工作太久，所以導致了現在頭痛的結果，故空格處應填現在完成時或現在完成進行時，即答案選C。

22. Professor James will give us a lecture on the Western culture, but when and where ______ yet.

A. hasn’t been decided                               B. haven’t decided    

C. isn’t being decided                                D. aren’t decided

分析：A。根據句意，此處應用被動語態，不用主動語態; 再根據句末的yet可知，最好用現在完成時。故選A最佳。

23. So far this year we ______ a fall in house prices by between 5 and 10 percent.

A. saw                      B. see                       C. had seen              D. have seen

分析：D。短語so far意為“到目前為止”，它習慣上要與現在完成時連用。

24. —Do you think we should accept that offer?

—Yes, we should, for we ______ such bad luck up till now, and time ______ out.

A. have had; is running                              B. had; is running     

C. have; has been run                                 D. have had; has been run

分析：A。根據句中的up till now可知，第一空應填現在完成時，故可排除B和C；run out的意思是“（時間）流逝”，為不及物動詞，以時間名詞作主語時，沒有被動語態，故可排除D。

25. —Have you known Dr. Jackson for a long time?

—Yes, since she ______ the Chinese Society.

A. has joined             B. joins                     C. had joined            D. joined

分析：D。since she join the Chinese Society為I have known Dr. Jackson since she join the Chinese Society之省略。連詞since所搭配的時態通常是：主句用現在完成時，從句用一般過去時。