Which characteristic do immature ecosystems have? 

A)  simple food webs 
B)  high species diversity 
C)  many specialized niches 
D)  numerous decomposers

Tall smokestacks and smaller industrial plants in urban areas are beneficial because they reduce local and regional air pollution.

A high population density in urban areas can increase the spread of ______________________________ if the infrastructure is inadequate and fails to provide sufficient drinking water and sewage treatment.

In which atmospheric layer does most of the Earth's weather occur? 

A)  the mesosphere 
B)  the thermosphere 
C)  the stratosphere 
D)  the troposphere