题目选项分析

In the arid land along the Colorado River, use of the river's water supply is strictly controlled：farms along the river each have a limited allocation that they are allowed to use for irrigation.But the trees that grow in narrow strips along the river's banks also use its water.Clearly, therefore, if farmers were to remove those trees, more water would be available for crop irrigation.

Which of the following, if true, most seriously weakens the argument?

【选项】The distribution of water allocations for irrigation is intended to prevent farms farther upstream from using water needed by farms farther downstream.