These chairs may look good but they’re very un-comfortable .A. to sit onB. to be sat onC.

问题补充：

These chairs may look good, but they’re very un-comfortable .

A. to sit on

B. to be sat on

C. sit on

D. sitting on

答案：

(答案→)A. to sit on

B. to be sat on

C. sit on

D. sitting on

解析：这里they是句子的主语，并且作介词on的宾语。这时用不定式的主动形式表示被动含义。故选A。句意：这些椅子看起来好，但它们坐上去不舒服。