Which of the following is true regarding units of geologic time?
a. eras are divided into epochs; epochs are divided into periods.
b. epochs are divided into periods; periods are divided into eras.
c. periods are divided into epochs; epochs are divided into eras.
d. eras are divided into periods; periods are divided into epochs.

d. Eras are divided into periods; periods are divided into epochs. The geologic time is divided from the largest defined unit of time called supereon, which is composed of eons. Eons are further subdivided into eras, periods, epochs and the smallest unit of time - ages.

The higher you go up the less oxygen there is 

The return of the wolf to yellowstone national park will probably affect the bird population to some degree, not significantly. while wolves will hunt birds when necessary, they prefer to hunt larger prey. the bird population will most likely only be affected when hunting is scarce for the wolves. birds of prey will also probably not be affected as much as other birds who spend more time on the ground hunting for food as the tend to spend more time in the air or in trees since they do not need to stay on the ground for long due to the nature of the way they hunt.