Which discovery was essential to the concept that all life forms have cells as basic units?

Which of the following areas has the highest population density?

All of the following statements about the biosphere are true except: a. light and heat from the sun do not affect the biosphere. b. the biosphere gets resources from earth's crust, hydrosphere and atmosphere. c. like "biology", the name for the biosphere comes from the greek word for life. d. the biosphere includes all living organisms on earth's surface.

In plate tectonics, why is the north american plate tectonically inactive?