Elements of the atmosphere

The correct answer is A. Nitrogen is the most abundant permanent gas in the atmosphere and dilutes oxygen. It is also essential to life, but most organisms cannot use atmospheric nitrogen gas directly. Nitrogen fixation and the nitrogen cycle convert it into compounds that organisms can use to build substances such as proteins and nucleic acids.

Carbon dioxide exchanges continuously between the atmosphere and the oceans. When seawater absorbs CO₂, some atmospheric carbon is stored in dissolved forms such as bicarbonate and carbonate, although this process also contributes to ocean acidification. Therefore option A correctly describes its role in the carbon cycle. The other options confuse carbon exchange with oxygen storage, cloud formation or direct ozone-layer thickening.

A is correct. In cold clouds, some particles can provide surfaces that help supercooled water freeze or water vapour form ice. Such particles are called ice nuclei. Not every atmospheric particle can do this; the effect depends on the particle and the cloud conditions.

Answer: A. Natural dust and human-made smoke are both atmospheric mixtures that may reduce visibility, but they do not necessarily have the same source or composition. Natural dust can come from soil, dry regions, exposed surfaces or wind erosion. Human-made smoke commonly comes from combustion in vehicles, factories, fires or household fuels and may contain fine particles and gases formed during burning. Different composition can mean different toxicity, particle size, atmospheric lifetime, effects on radiation and cloud processes, and control measures. B is wrong because source and composition are not identical. C is false because smoke is released into and transported through the atmosphere. D is too absolute: natural dust can be harmful in some conditions, but it is not always toxic. Proper distinction improves monitoring, health assessment and pollution control. Memory cue: identify the source before judging the risk.

Atmospheric gases, water vapour and suspended particles interact with one another and with the land, oceans, ice and living organisms. Water vapour links the atmosphere with the hydrological cycle, gases influence radiation and climate, and particles affect clouds, air quality and health. These exchanges show why the atmosphere is an integrated Earth-system component. Option B denies these interactions, C incorrectly excludes gases, and D contradicts the atmosphere–hydrosphere relationship.

A is correct. Nitrogen makes up most of dry air and dilutes oxygen, so air supports less intense combustion than a mixture with a much higher oxygen proportion. Nitrogen also enters the nitrogen cycle after fixation converts atmospheric N₂ into compounds such as ammonium or nitrate that organisms can use. Most plants cannot use atmospheric N₂ directly.

Studying these gases together shows that atmospheric composition is a connected system rather than a list of isolated percentages. Oxygen supports respiration and combustion, carbon dioxide is used in photosynthesis and influences the greenhouse effect, while water vapour drives condensation, clouds and rainfall and also affects heat balance. Thus option A correctly expresses their integrated importance; the remaining options wrongly exclude major functions.