The correct option is (c) 1 and 3 only.
Explanation
Air is a mixture of various gases, water vapor, and particulate matter. While the proportions of major gases are relatively stable in the lower atmosphere, the overall composition can exhibit spatial and temporal variations.
Statement 1 is Correct: Nitrogen and Oxygen are the dominant constituents of the atmosphere. Nitrogen accounts for approximately 78% and Oxygen for approximately 21% by volume. Together, these two gases comprise about 99% of clean, dry air.
Statement 2 is Incorrect: The remaining 1% of the air is composed primarily of Argon (approximately 0.93%). Carbon dioxide constitutes only about 0.03% to 0.04%. Therefore, Argon constitutes the majority of the remaining fraction, not Carbon dioxide.
Statement 3 is Correct: The composition of air varies geographically and temporally. For example, the concentration of water vapor fluctuates significantly, ranging from nearly 0% in arid regions to about 4% in humid tropics. Additionally, the levels of industrial pollutants (such as oxides of sulphur and nitrogen) and suspended particulate matter vary between urban/industrial areas and rural environments.
Key Takeaway: The standard composition of dry air by volume in descending order is Nitrogen > Oxygen > Argon > Carbon Dioxide.