But even if uncooperative farmers were to join in the conservation efforts, this would only delay the depletion of the aquifer.

将上面一句话插入下述A、B、C、D四个空缺中。

The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water by irrigating less frequently or by switching to crops that require less water. A■Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. B■The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the process are drawing down the entire region’s water supplies. C■In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to transport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. D■

This economic reliance on livestock in certain regions makes large tracts of land susceptible to overgrazing.

将上面一句话插入下述A、B、C、D四个空缺中。

A■The raising of livestock is a major economic activity in semiarid lands, where grasses are generally the dominant type of natural vegetation. B■The consequences of an excessive number of livestock grazing in an area are the reduction of the vegetation cover and the trampling and pulverization of the soil. C■ This is usually followed by the drying of the soil and accelerated erosion. D■