In the last few decades, grassy wetlands,

问题：

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.