In the atmosphere, 50% of the air lies below an altitude of

Which description provides the best objective summary of the article in the passage?

The greenhouse effect a. something that has occurred for millions of years b. an unnatural phenomenon caused by humans that maintains earths temperature range. c. the results of the differences in the angle of the suns rays d. an unnatural phenomenon that causes heat energy to be radiated back into the atmosphere

This is an active transport mechanism by which cells pump sodium and potassium ions against the concentration gradient