单选题 20. The surface of Venus contains calcite, a mineral that absorbs the gas sulfur dioxide. Sulfur dioxide is often produced by volcanic activity and is plentiful in Venus' atmosphere, so there must be a source, perhaps volcanic, producing sulfur dioxide on Venus.
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.

【正确答案】 A

【答案解析】 A：选项取反。在过去，金星二氧化硫的水平远远高于现在。这说明现在二氧化硫少了，这在一定程度上削弱了火山是二氧化硫的来源这一论证，削弱原文，正确。
B：该选项本身就在削弱原文，不选。
C：本文讨论的是二氧化硫的来源，与观察是否容易无关，不选。
D：本文讨论的是金星，和地球无关，不选。
E：本文讨论的是现状，即明明金星表面有方解石吸收二氧化硫，可是金星大气层中依然有二氧化硫，与未来无关，不选。