Answer: A. Dry air is approximately 78% \(\text{N}_2\) and 21% \(\text{O}_2\) by volume. The next most abundant component is argon at about 0.93%. The other noble gases are present only in trace amounts:
• Neon ~0.0018%
• Helium ~0.0005%
• Krypton ~0.0001%
Argon is roughly 500× more abundant than neon, and ~5000× more abundant than helium or krypton — by far the dominant noble gas in dry air.
Answer A. (Useful chemistry-memory hook: argon is also the third most abundant gas in dry air overall, after \(\text{N}_2\) and \(\text{O}_2\).)
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