新概念英语第二册的第77课中的一句话
The operation,which lasted for over four hours,proved to be very difficult because of the hard resin which covered the skin.
proved to be very difficult,to在这里是作什么用,另外be加形容词是不是表示一种状态?
另外那课后练习：It was difficult because of the hard resin ____the skin.这里为什么是covering,而不是covered?