all work is pleasant ----when the habit of working is formin

选择C
解释：这是典型的主动表被动的句型：sth+ be+ adj + to do sth
如：
Chinese is easy to learn. （汉语容易学.）

English is difficult to learn. （英语难学.）

The chair is comfortable to sit on. （那张椅子坐上去很舒服.）