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

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→)A. to sit onB. to be sat onC. sit onD. 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。

好好学习下