Read the following passage carefully and answer the questions given below :
Low-lying flat surfaces with very gentle slopes are called plains. Some plains are extremely level, while others have an undulating (wave like) surface.
Most plains have been formed by rivers and their tributaries. (A tributary is a river flowing into a larger river). The rivers which flow down mountains and plateau was away some material form the mountains and plateaus. When the rivers reach gentle slopes, they slow down and material they carry gets deposited on the plains. The deposited material, comprising sand, silt and clay, is called alluvium.
(i) What are plains ?
(ii) What are tributaries ?
(iii) What do rivers flowing down mountains and plateaus wash away?
(iv) Find a word in the passage which means 'not steep'.
