What is the approximate composition of today's atmosphere?

The composition of today’s atmosphere is predominantly made up of nitrogen and oxygen, with nitrogen making up approximately 78% and oxygen about 21%. This means that choice B, which states 80% nitrogen and 20% oxygen, is the closest approximation to the actual composition.

Understanding this is important because nitrogen acts as a stable background gas and does not easily react, while oxygen is essential for combustion and respiration. The other options reflect percentages that don't align closely with the scientific understanding of our atmosphere. For instance, 70% nitrogen and 30% oxygen would indicate a higher oxygen content than is actually present, while 90% nitrogen and 10% oxygen significantly underrepresents the oxygen levels. Additionally, a 50/50 split would not reflect the actual dominant presence of nitrogen. Therefore, the approximation given in choice B effectively captures the balance of gases found in the atmosphere today.