Modern methods of tracking polar bear populations have been employed only since the mid-1980s, and are expensive ________ (perform) consistently over a large area.
横线内为什么是to perform 而不是 to be performed
"现代方法"不是被执行的吗?
Modern methods ....have been employed 是被动,现代技术被使用
Modern methods are expensive to be performed 就不可以?
还有adj如interesting,difficult等都是此类用法
像什么easy,hard,pleasnt也是
在高中语法中会有提及
即主动形式表示被动意思