It is in urban areas today where seventy-five percent of the world’s population lives. most of the land is used for housing, transportation, commercial interests, and manufacturing. one-quarter of each city’s land is still being used for farming. most residents either grow, harvest, or sell crops.

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