question: Approximately 20% of the atmosphere is made of which element?
distractor3: nitrogen
distractor1: hydrogen
distractor2: carbon
correct_answer: oxygen
support: Approximately 20% of the atmosphere is oxygen. This gas is essential for life. In environments where oxygen is in low supply, it can be provided from a tank. Since gases are very compressible, a large amount of oxygen can be stored in a relatively small container. When it is released, the volume expands and the pressure decreases. The gas is then available for breathing under normal pressure.