.The chair looks rather hard but believe it or not; it is very comfortable .A. sittingB.

问题补充：

.The chair looks rather hard, but believe it or not; it is very comfortable .

A. sitting

B. to sit

C. sitting on

D. to sit on

答案：

A. sitting

B. to sit

C. sitting on

(答案→)D. to sit on

解析：不定式做定语与所修饰的词为动宾关系时，如果动词为不及物动词时要在后面接上合适的介词使之相当于及物动词的性质。所以选D。