—How about joining us in the summer camp? —I wish I could I have to practice for the pia

问题补充：

—How about joining us in the summer camp?

—I wish I could, I have to practice for the piano test.

A. because

B. so

C. but

D. although

答案：

A. because

B. so

(答案→)C. but

D. although

解析：because因为；so因此；but但是；although 虽然，尽管。句意：加入我们的夏令营怎么样？——但愿我能，但是我必须得为钢琴比赛练习。结合语境可知上下文是转折关系，故选C。