LSAT阅读一道，求解释。（已简化）

A second achievement of the judicial system is the protection that has been provided against encroachment by the states into tribal affairs. Federal judges are not inclined to view favorably efforts to extend states powers and jurisdictions because of the direct threat that such expansion poses to the exercise of federal powers. In the absence of a federal statute directly and clearly allocating a function to the states, federal judges are inclined to reserve for the federal government—and the tribal governments under its charge—all those powers and rights they can be said to have possessed historically.

答案：(D) detrimental to the interests of Native Americans

问：

1）从哪句话推出D?

2） 文中说因为such expansion poses threat to the exercise of federal powers.这个THREAT段落中指出了么？EXERCEISE OF FEDERAL POWERS是什么意思？它怎么威胁到这个POWERS的？