30. He was very busy yesterday, otherwise he ______ to the meeting. A. would come B. came C. would have come D. had come 解析：(C)otherwise用来表示一种含蓄的虚拟假设，因为是对过去(yesterday)的情况的假设，所以应该用would have come形式。

29. It is time that the government ______ measures to protect the rare birds and animals. A. takes B. took C. has taken D. taking 解析：(B)It is time that是一种要求从句用虚拟语气的常见句型。虚拟语气形式为动词的过去式。

28. He told her to return the book in time so that others ______ a chance to read it . A. may have B. will have C. would have D. might have 解析：(D)连so that引导的从句中要求用虚拟语气，表示一种假设，形式为might+动词原形。

27. Without electronic computers, much of today’s advanced technology ______ . A. will not have been achieved B. have not been achieved C. would not have been achieved D. had not been achieved 解析：(C)这是一人含蓄虚拟条件句，通过介词短语without,表示潜在的一种假设，而主句中的情况也早已经发生，所以是对过去情况的一种假设，因此用would not have been achieved这一形式。

26. Although most adopted persons want the right to know who their natural parents are, some who have found them wish that they ______ the experience of meeting. A. hadn’t B. didn’t have had C. hadn’t had D. hadn’t have 解析：(C)who引导的定语从句中的现在完成时表示动作业已发生，所以主句中表示的虚拟是针对已经发生了的行为，所以应该用过去完成式表示对过去的虚拟。

25. Most insurance agents would rather you______ anything about collecting claims until they investigate the situation. A. do B. don’t C. didn’t D. didn’t do 解析：(D)would rather后接从句要求用虚拟语气，形式为过去式，表示对现在或将来的假设，或为过去完成式表示对过去的情况的假设。

24. He described the town as if he ______ it himself. A. had seen B. has seen C. saw D. sees 解析：A．as if引导的与过去相反的虚拟。

23. John is so strongly built that he looks as if he ______ as elephant. A. lifts B. is lifting C. lifted D. could lift 解析：D．as if引导的与现在事实相反的虚拟。

22. Looking round the town, he felt as though he ______ away for ages. A. has been B. was C. is D. had been 解析：D．felt是过去式，所以该句是由as though引导的与过去相反的虚拟。

21. As usual, he put on a show as though his trip______ a great success. A. had been B. has been C. were D. was 解析：A．as though引导的与过去相反的虚拟。