This kind of chair looks hard ,but in fact it is comfortable __.A.to sit in B.to sit C.sitting on

This kind of chair looks hard ,but in fact it is comfortable __.A.to sit in B.to sit C.sitting on
我个人感觉是选A ,..
说什么is是动词,comfortable和 坐 是同时发生的,很不理解...
我想问一下为什么选C