TOEFL Listening: TPO-TOEFL听力TPO - WS7B7X1F1HZ35SLGR$

Before the use of uranium-lead analysis, where did most geologists think the Grand Canyon sandstone came from? A. An ancient lake located in the American Southwest B. A desert that once connected two continents C. Sands carried by a river from the Appalachian Mountains D. A nearby mountain range that had flattened out over time