Ⅰ.单句语法填空
1．(2019·全国卷Ⅰ)Modern methods of/for tracking polar bear populations have been employed only since the mid1980s, and are expensive to perform consistently over a large area.
解析：考查介词。 此处表示追踪北极熊数量的现代方法，表示“……的”，故答案为of；也可理解为目的，故答案也可为for。
2．(2019·全国卷Ⅲ)We were first greeted with the barking by a pack of dogs, seven to be exact.
解析：考查介词。a pack of为固定搭配，表示“一包……；一群……”，故填of。
3．(2019·北京卷)First celebrated in 1970，the Day now includes events in more than 190 countries and regions (地区)．
解析：考查介词。表示“在哪一年”用介词in，故填in。
