Which of the following gases was likely absent in significant amounts in the Earth's early atmosphere?

Oxygen was likely absent in significant amounts in the Earth's early atmosphere due to the lack of biological processes that produce it, such as photosynthesis. The early atmosphere was primarily composed of gases released from volcanic activity, which included carbon dioxide, methane, ammonia, and water vapor. Over time, as the planet evolved and life forms, especially photosynthetic organisms, began to emerge, oxygen levels started to increase dramatically. This process is known as the Great Oxygenation Event. Therefore, the absence of significant amounts of oxygen in the Earth's early atmosphere is a key point in understanding the development of our planet's environment.