How has the atmosphere changed over time? Choose one: A. Earth's atmosphere has remained mostly constant over time. B. The primary component of the early atmosphere was carbon dioxide. It now consists primarily of oxygen with small amounts of nitrogen. C. The oxygen-rich atmosphere became enriched in nitrogen over time. Carbon dioxide was added after photosynthetic plants evolved. D. Earth's atmosphere has transformed from a toxic mix of methane, carbon dioxide, and ammonia to a habitable mix of 78% nitrogen ( N2) and 21% oxygen (O2)