Covers GAAP

Direct answer: B, carbon dioxide. Carbon dioxide is present in a very small proportion of the atmosphere, well below 1% in the usual school-level description. Even at this low concentration, it is important in two different ways. Green plants take in carbon dioxide as a raw material for photosynthesis, and the gas absorbs some outgoing infrared radiation, contributing to the natural greenhouse effect. Option A is wrong because nitrogen is the most abundant gas, about 78%. Option C is wrong because oxygen forms about 21%, not less than 1%. Option D is wrong because both nitrogen and oxygen are major gases, not trace gases. The useful warning is that a gas may be small in quantity but large in importance; concentration and function are not the same thing.

Direct answer: A is correct. Humidity means the presence or amount of water vapour in the air. Water vapour is invisible, but it changes how moist or dry the air feels and participates in cloud formation, condensation and rainfall. Relative humidity is a comparison: it expresses how much water vapour is actually present compared with the maximum amount the air could hold at that temperature. Warm air can hold more vapour than cold air, so a temperature change can alter relative humidity even without adding or removing much water. B is wrong because humidity is not a colour. C is wrong because it has nothing to do with Earth’s metallic layers. D is wrong because dust is particulate matter, not humidity. Memory cue: humidity means moisture in air; do not confuse it with dust or temperature alone.

Answer: A. The ozone hole refers to a marked seasonal thinning of stratospheric ozone, especially over polar regions. Ozone is made of three oxygen atoms and is concentrated mainly in the stratosphere. It absorbs a large portion of harmful ultraviolet radiation from the Sun, particularly UV-B. When stratospheric ozone is depleted, more ultraviolet radiation can reach Earth’s surface, increasing risks to living organisms. A is correct because the issue concerns stratospheric ozone, not the complete disappearance of all ozone. B and C are wrong because nitrogen and argon are major atmospheric gases but are not the atmospheric substances whose depletion defines the ozone-hole problem. D is wrong because sea salt is an aerosol, not the protective gas layer being depleted. Common confusion: ozone near the ground can be a pollutant, while stratospheric ozone protects life from ultraviolet radiation; the location matters.

Answer: A. Monsoon circulation commonly transports moist maritime air from warm oceanic areas toward land. Warm surfaces receive energy that promotes evaporation from the sea, soil and vegetation, and the winds carry the resulting water vapour over large regions. When this moist air rises and cools, condensation produces clouds and may lead to rainfall. A is correct because it combines the two important causes: a moisture source and atmospheric transport, supported by warm conditions that increase evaporation. B is wrong because monsoon winds are not characterised only by dry air. C is wrong because the Sun continues to rise during the monsoon season; cloudiness does not mean the Sun disappears. D is wrong because oxygen does not vanish from the atmosphere. Memory cue: monsoon moisture comes mainly from warm water surfaces plus winds that transport the vapour.

Direct answer: The mesosphere is roughly between 50 and 80 kilometres above Earth’s surface, so option A is correct. The layer begins at the stratopause, near 50 km, and ends at the mesopause, near 80 km. In the mesosphere, temperature generally decreases with height, and many meteoroids become luminous or burn because they collide with atmospheric particles. Option B, 0–13 km, describes the approximate troposphere. Option C, 80–400 km, includes much of the thermosphere rather than the mesosphere. Option D is far outside the usual school-level range for the mesosphere. Exact boundaries can vary somewhat, but 50–80 km is the standard approximate range used in textbooks. Memory cue: remember the sequence of approximate tops: troposphere 13 km, stratosphere 50 km, mesosphere 80 km.

A is correct. At higher altitudes, less air lies above a location, so the overlying air weighs less and pressure generally decreases. Weather can cause local pressure variations, but it does not change this usual relationship with altitude. Air pressure exists wherever there is an atmosphere, over land as well as oceans.