<em><u>A </u><u>greenhouse</u><u> gas is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.</u></em>
Answer:Geographic featured that acted as norther borders of the Roman Empire at its height were: Rhine River, Danube River, Carpathian Mountains, and the Black Sea.