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The correct answer identifies the small proportions of gases found in today’s atmosphere alongside the major components, nitrogen and oxygen. Carbon dioxide and water vapor are indeed present in minor amounts compared to nitrogen, which makes up about 78% of the atmosphere, and oxygen, which comprises about 21%.
Carbon dioxide, although it is only around 0.04% of the atmosphere, is critical for life on Earth as it plays a vital role in the greenhouse effect and is used by plants for photosynthesis. Water vapor varies greatly but often accounts for up to 4% of the atmosphere, contributing significantly to weather and climate.
Noble gases, which include argon and trace amounts of others like neon and krypton, are also present in very small proportions. Argon is actually the third most abundant gas, making up about 0.93% of the atmosphere, while the other noble gases are present in trace amounts.
The other answer choices list gases that are not typically considered among the small proportions in the atmosphere. Heavy metals and hydrocarbons are not atmospheric gases; they refer to pollutants or particulate matter. Ozone, while important in the stratosphere for blocking UV radiation, is also not a component of the atmosphere in significant amounts by