If old mountains are worn down over the course of millions of years, what is one of the primary reasons that mountains still exist? A. New mountains are created through city development. B. New mountains are created from leaf litter. C. New mountains are created through tectonic and volcanic activity. D. New mountains are created from decomposition of plants and animals.

Which of the following statements best explains the relationship between medicines and concerns about the shrinking of the amazon rainforest?

Ozone in the upper atmosphere has been depleted by a. sulfur dioxide. b. chlorofluorocarbons. c. carbon dioxide. d. oxygen.

Some gamma ray bursts are hypothesized to come from mergers of neutron stars or black holes. if this hypothesis is correct, what else should we in principle be able to detect from such mergers? a. gravitiational waves b. x-ray bursts c. significant disruption of gas thousands of light-years away in the surrounding galaxy d. an "afterglow" of bright visible light, lasting for weeks