According to the processes described in paragraph 1, what is the relationship between radioactivity and the steam produced by geothermal heat? A. Geothermally heated steam is produced when water is exposed to radioactivity deep underground. B. When water is introduced into holes drilled thousands of feet in the ground, it becomes radioactive and turns to steam. C. Radioactivity heats Earth's interior rock, which in turn can heat water to the point it becomes steam. D. When a reservoir of steam in subsurface rock is produced by radioactivity, it is said to be geothermally heated.