第15题: [单项选择]It can be concluded from this passage about the Grand Canyon that ______. A. its shapes are constantly changing B. it contains approximately 14 million tons of rock C. its eruptions have increased in recent years D. it is being eroded by toxic waste and pollutants 参考答案:A 答案解析:最后一段“从那以后,大峡谷的形成日积月累,除了河流的侵蚀作用外,还有其他因素,如冷热、雨雪以及岩石的各种阻力增加了侵蚀的可能性。大峡谷壁面塌落,河流获得了切割工具。成吨的碎石和从高原上流下的雨水产生的