accounting principles

Dust particles act as condensation nuclei in the atmosphere. Water vapour generally needs a tiny surface on which it can condense; dust, salt and other aerosols provide that surface. The droplets may then grow and join to form clouds. Hence option D is correct. Dust does not build mountains, produce earthquakes or directly create ocean tides.

Answer: A. Photosynthesis and respiration show the linked movement of carbon dioxide and oxygen in living systems. During photosynthesis, green plants use carbon dioxide and water, with light energy, to make food and release oxygen. During respiration, plants, animals, and many other organisms use oxygen to release energy from food and produce carbon dioxide and water. The two processes are not exact opposites in every detail, but together they clearly demonstrate the biological exchange of these gases. B, C, and D are wrong because earthquakes, volcanoes, tides, magnetism, and eclipses are physical or geological phenomena and do not explain the regular biological gas exchange. Memory cue: “Plants take in CO2 and give out O2; respiration largely reverses the gas exchange.”

The correct answer is A: nitrogen and oxygen. Dry air means air considered without variable water vapour. Nitrogen makes up about 78 percent of dry air and oxygen about 21 percent, so together they account for approximately 99 percent. The figures are rounded, but the important idea is that these two gases dominate the composition of dry air. B is wrong because carbon dioxide and ozone occur in very small amounts. C is wrong because water vapour is excluded when we specify dry air, and dust is not a principal gaseous component. D is wrong because methane and neon are trace gases present in extremely small concentrations. A useful caution is that the composition of moist air changes when water vapour is included, but the standard “about 99 percent” statement refers to dry air. Memory cue: “78 plus 21 gives about 99: nitrogen plus oxygen.”

A, the troposphere. Gravity keeps most atmospheric mass close to Earth’s surface, and the troposphere is the lowest major layer. The weight of the air above produces greater pressure near the surface, while the greater concentration of air molecules gives this layer higher density than the layers above it. Both pressure and density generally decrease with altitude.