问题补充：
During the tourist season, there are many people wandering in this city to
see the old castles in the sixteenth century.
A. to be built
B. being built
C. having been built
D. built
答案：
A. to be built
B. being built
C. having been built
(答案→)D. built
解析：非谓语动词的动作先于句中谓语动词的动作时， 用过去分词。
