Though they are constantly changing, ecosystems are naturally a-balanced b-crowded c-consistent d-species-rich

A-balanced

Explanation:

An ecosystem is the community where living beings interact with their non-living environment so as to sustain and survive. The dynamics and structure of the ecosystem changes according the abiotic (non-living) and biotic (living)  factors.

The ecosystem remain balanced due to these changes. For example natural calamity such as flood being an abiotic factor will be responsible for controlling the population of overcrowded habitat. Also the population of predators will control the growth and abundance of prey species.  

answer: the exterior temperature of commercial jet planes as they leave the stratosphere and return to the ground increases steadily.

explanation:

the lower most layer of atmosphere is troposphere which extends from 0 to 12 km above the surface of the earth. the temperature initially decreases as distance is increased from the earth's surface. commercial jets fly in lower part of the stratosphere. the stratosphere extends upto 50 km. the temperature in stratosphere starts rising again due to presence of ozone which traps the uv rays.

thus, when the planes leave the stratosphere and return to the ground, the exterior temperature increases steadily.