多项选择题 下列（）项影响煤层瓦斯含量 A. 煤田地质史 B. 地质构造 C. 煤层赋存条件 D. 煤的变质程度

单项选择题 The ideal gas goes through the cycle process as shown in the solid line in the figure. The two isochoric lines are respectively tangent to the cycle process curve at points a and c.The two isothermal lines are tangent to the cycle curve at points b and d. a, b, c and d divide the cycle into four stages: ab, bc, cd and da. Therefore, the exothermic stage in the four stages can be identified as ________ from the figure.

单项选择题 将一枚硬币重复掷n次，以X和Y分别表示正面向上和反面向上的次数，则X和Y的相关系数等于    (A) -1．    (B) 0．    (C) ．    (D) 1．