Which of the following descriptions are true? A: A.China has both marine and land boundary line of more than 20,000 kilometers and a coastline of 18,000 kilometers. B: B. China almost has all kinds of landforms, such as plateau, plain and basin. C: C. China occupys the third large geographical area in the world next only to Russia and America. D: D. The land surface of China slopes down from west to east in a three-step staircase.