How is the problem that the surface may "sink" related to the problem that water heated geothermally may contain "toxic materials"? A. Both problems could be solved by returning groundwater that is removed from an underground heat reservoir back to the reservoir after heat is extracted from it. B. The problem of sinking is more difficult to solve than is the problem of toxic materials. C. Land at the surface sinks because the rock beneath the surface is weakened when salts and toxic materials are removed from it in the process of extracting geothermal energy. D. Both problems are caused by the fact that the hot groundwater in a heat reservoir dissolves the rock, which weakens the rock and makes the water toxic with salt.