What role do bacteria play in the nitrogen cycle?

a. they fix nitrogen into forms usable by plants
b. they release fixed nitrogen into the atmosphere
c. they move nitrogen through the vascular system
d. they make nitrogen available for humans

Which statement describes how favorable traits in a population relate to natural selection? they are the only traits that ever exist in the population. they build in the population over time. they are rarely passed on to offspring. they are found only in a few individuals within the population.