Which of the following is an assumption on which the argument depends? A. In the recent past, the level of sulfur dioxide in Venus' atmosphere was not much higher than it is now. B. If there is volcanic activity on the surface of Venus, it is not adequate to maintain the observed amount of sulfur dioxide in the atmosphere. C. The sulfur dioxide in Venus' atmosphere forms clouds that prevent direct observation of the surface. D. Sulfur dioxide produced by volcanic activity on the Earth's surface is also absorbed by minerals other than calcite. E. The calcite on the surface of Venus is in sufficient supply to continue to absorb sulfur for millions of years to come.