答案的确是operating on.
如果是to- - - - ,你就必须改写成：
- -- and he needed to be operated on.
记住这个结构：
The work needs / wants / requires TO BE DONE again
== The work needs / wants / requires DOING again填原型operate 行吗？不行，但可以这样：and the doctors needed to operate on him. (换一换主体对象)