8. a narrow region between two air masses of different densities is a a. ridge. b. system. c. gulley. d. front.

The answer is d. front. A front is the narrow region between two air masses of different densities. The interaction of the air masses cause the mass that is less dense to rise. Once air rises it cools and condenses to form clouds and eventually leads to precipitation. Using cold fronts as an example, the denser mass of cold air forces the lighter warm air to rise.

They are all in a iving organism.