Which is a challenge that is most likely unique to urban areas?
a. power outages
b. shortage of water
c. population density
d. lack of personal time?

Lack of personal time, water shortages, and power outages may be problems in any locale. Population density, however, is a problem usually associated with life in urban areas.

they are non-matter

explanation:

they are non-matter. basically, any type of energy or any abstract concept is an example of something that does not matter. other examples include dreams, magnetism, radio, and hate