5. Mining has polluted mountain streams in western U. S. watersheds and accounts for about half of all the country's emissions of toxic chemicals.

a. True
b. False

a. True

Explanation:

Mining is defined as the process of extraction of minerals from Earth. Mining is one of the major reason of water pollution as it affects water in processing ore and allows the discharge of  mine effluent into the rivers or sea.

Mining in U.S. is active from very long time and it extract or process several minerals such as coal, metals such as copper,  iron, and industrial minerals.

The given statement is true,  western U.S. watersheds are majorly affected by mining as per the report of EPA and mining became biggest source of toxic pollution.

Hence, the correct answer is "a. True".