Modern methods —— tracking polar bear populations have been employed only since the mid-1980s, and are expensive to perform consistently over a large area. 2、Of —— nine recognized polar bear subpopulations, three are declining, six are stable,one is increasing, and nine lack enough data. 3、I work not because I have to, —— because I want to. 4、When we got a call saying she was short-listed, we thought it was —— joke. 5、We were first greeted with the barking by a pack —— dogs, seven to be exact. 6、Everybody wears —— same style of clothes.But can uniforms help improve school standards? 7、The answer —— this question is not clear. 8、While running regularly can’t make you live forever, the review says it is more effective at lengthening life —— walking, cycling or swimming. 9、If you are time poor, you need run for only half the time to get the same benefits as other sports, so perhaps we should all give —— a try. 10、Corn production has jumped nearly 125 percent over —— past 25 years, while rice has increased only 7 percent.The government encourages farmers to grow corn instead of rice to improve water quality. 11、Corn uses less water —— rice and creates less fertilizer (化肥) runoff. the first, the last, the next, the past,