I. 1.(语法填空)Modern methods________ tracking polar bear population have been employed only since the mid-19890s,and are expensive to perform consistently over a large area.
2.(语法填空)We were first greeted with the barking by a pack ________ dogs,seven to be exact.
3.(语法填空) But can uniforms help improve school standards? The answer______this question is not clear.
4.(单句改错)At the end of our trip, I told my father that I planned to return for every two years,and he agreed.
5.(单句改错) Still I was unwilling to play the game for them sometimes.
