Although the play had already been on for 10 minutes I was still unable to find a seat .

问题补充：

Although the play had already been on for 10 minutes, I was still unable to find a seat .

A. to sit

B. for sit on

C. to sit on

D. for sitting

答案：

A. to sit

B. for sit on

(答案→)C. to sit on

D. for sitting

解析：考查不定式做定语，如果不定式的动词是不及物动词，这题不定式修饰a seat，要加相应的介词，sit on the seat，句意：虽然戏剧已经开始10分钟了，我还找不到座位坐下。选C。