语法填空
（1）Modem methods of tracking polar bear populations have been employed only since the mid-1980s, and are expensive ________ (perform) consistently over a large area.
（2）Once his message was delivered, he allowed me __________ (stay)and watch.
（3）You don’t have to run fast or for long ______ (see) the benefit.
（4）Fat and salt are very important parts of a diet. They are required ________ (process) the food that we eat, to recover from injury and for several other bodily functions.