What is the current greatest threat to agricultural sustainability? a. erosion b. water pollution c. air quality d. all of the above (not d)

The current greatest threat to agricultural sustainability is A. erosion.
Given that erosion is affecting the soil by breaking it apart, and given that soil is rather important for agriculture, if something happens to it, the agriculture of that particular area will be affected as well. Although air quality and water pollution are definitely important, they are not as dangerous as erosion.

The troposphere gets warmer as altitude increases.

This type of galaxy is an approximately ellipsoidal shape and a smooth, nearly featureless image. they are the largest single star systems, in the universe and are built by the collisions of many smaller galaxies.