Important Terminology

Correct answer: D. A trace gas is present in a relatively small concentration, but small quantity does not mean zero importance. Such gases can absorb or emit particular wavelengths of radiation, take part in atmospheric chemical reactions and influence air quality. For example, ozone has important radiative and chemical roles, while some other trace gases affect warming or pollution chemistry. Their effects depend on their properties, not simply on how much of them exists. A is wrong because trace gases can have strong effects despite low concentration. B is unrelated: atmospheric gases do not merely change Earth’s shape. C is false because trace gases do not all disappear permanently. The useful warning is: concentration and importance are different ideas.

Correct answer: A. In the atmosphere, altitude means greater distance above Earth’s surface. As altitude increases, the column of air above a place becomes smaller, so atmospheric pressure generally falls. Air also becomes thinner, so density generally decreases. Temperature does not follow one single direction through all atmospheric layers: it decreases in the troposphere, increases in the stratosphere, decreases in the mesosphere, and increases in the thermosphere. Therefore, temperature trend is an important clue for identifying layers. B is wrong because pressure does not increase upward and temperature is not always decreasing. C is wrong because the exosphere contains extremely thin gas. D is wrong because temperature is useful for distinguishing layers. Memory cue: pressure and density thin upward; temperature changes direction by layer.