A. the atmosphere changed dramatically B. oxygen levels on the earth became lower C. the sea levels became lower greatly D. the sediment gave off enormous carbon and methane 参考答案:D 答案解析:细节题 本题涉及2.5亿年前全球变暖,也就是温室效应在什么情况下发生的。根据第四段可知:海洋沉积层一旦暴露在大气中,沉积层中的大量碳和甲烷释放造成温室气体。这说明当时的温室效应是由释放到大气中的这些