问题补充：
The tourists gathered at was once a square, is used as a theme park.
A. where; which
B. what; which
C. which; what
D. that; which
答案：
A. where; which
B. what; which
C. which; what
D. that; which
解析：游客在曾经是广场现在是主题公园的地方集中。介词at后面的宾语从句缺主语，故第一空填what在从句中作主语；逗号后面是一个非限制性定语从句，故填which. 本题答案选B.
