In view of all sorts of rumors and guesses, experts carried out some careful and scientific research and_____ the possibility of a second earthquake in this area.
第二部分:英语知识运用(共两节,满分 45 分)
第一节:单项填空(共 15 小题;每小题 1 分,满分 15 分)
从 A、B、C、D 四个选项中,选出可以填入空白处的最佳选项。
21. Great changes ______ in China in the past 20 years.
A. took place B. have taken place C. were taken place D. have been taken place