water allocation

In the arid land along the Colorado River, use of the river's water 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?
a. The trees along the river’s banks shelter it from the sun and wind, thereby greatly reducing the amount of water lost through evaporation
b. Owners of farms along the river will probably not undertake the expense of cutting down trees along the banks unless they are granted a greater allocation of water in return
c. Many of the tree species currently found along the river’s banks are specifically adapted to growing in places where tree roots remain constantly wet.
d. The strip of land where trees grow along the river’s banks would not be suitable for growing crops if the trees were removed.
e. The distribution of water allocations for irrigation is intended to prevent farms father upstream from using water needed by farms father downstream
OA: A
But I wonder why D is not the answer since D also weakens the argument and how to compare with A to get the best answer.