单选题 In the last few decades, grassy wetlands, essential to the nesting and breeding of ducks, geese, swans, and most other species of waterfowl, have been extensively drained and cultivated in southern Canada and the northern United States, Duck populations in North American have plummeted during this time, but populations of swans and geese have been affected less dramatically.
Which of the following, if true, most helps to explain the difference mentioned above?

A、 Prohibition of hunting of waterfowl is easier to enforce in areas under cultivation than in wild lands.
B、 Most geese and swans nest and breed farther north than ducks do, in areas that still are not cultivated.
C、 Land that has been harvested rarely provides food suitable for waterfowl.
D、 Goose and swan populations decline in periods of drought, when breeding sites are fewer.
E、 (E) Because they are larger than ducks, geese and swans have a harder time finding protected nesting sites in areas that are cultivated.

【正确答案】 B

【答案解析】[解析] 本题为解释差异，抓住差异点来定位答案，解释为什么许多水鸟筑巢和孵化的地区被开发，鸭类数目急剧下降，但鹅类与天鹅数目未受影响。显而易见，解释差异需寻求差异双方，duck geese和swan，定位选项发现只有(A)、(E)符合。(E)与上面差异刚好相反；(B)指出由于geese与swan比duck居住在更靠北部的仍未被开发的地区，所以解释了差异，因此(B)正确。