问题：

He dressed up a clown
他打扮成一个小丑。
A.to B. for C. as

to 对人 for 对物 as 对职业

例：

This book is useful to students.
这本书对学生很有用。(学生是人 → to )

This city is famous for its food.
这座城市以美食闻名。(美食是物 → for)

He is famous as a writer.
他作为一名作家出名。(作家是职业 → as)