问题补充：
people went to the concert which was held in People’s Square the night of May Day.
A. Three thousand, in
B. Thousands, on
C. Many thousand, in
D. Thousands of, on
答案：
A. Three thousand, in
B. Thousands, on
C. Many thousand, in
(答案→)D. Thousands of, on
解析：hundred 、thousand这些量词前面有数字的，如two thousand ，thousand后面绝不能加s;，表示具体数字。如果后面加s，前面则不能有数字，表示一个模糊地数字概念。
在表示时间时in表示一个大致的时间范围，on则表示在具体的某一天。根据题意，前面人们的数量时模糊概念，只表示多。后面五一劳动节晚上是具体的某一天，所以选D。
