语法填空
（1）Modern methods ___of__ tracking polar bear populations have been employed only since the mid-1980s, and are expensive ________ (perform) consistently over a large area.
（2）On the last day of our week-long stay,we _were invited__(invite) to attend a private concert on a beautiful farm on the North Shore under the stars,________ (listen)to musicians and meeting interesting locals.
（3）The government encourages farmers to grow corn instead of rice ________ (improve) water quality.