TOEFL Reading: ETS-TOEFL阅读机经 - 1GWH61Y288R8JMM0L$

Why does the author ask the reader to "Imagine a winding stream" A. To explain how the presence of bars changes the speed and direction of water flow in a stream B. To explain why bars are more common than alluvial fans or other types of sediment deposits C. To illustrate the particular difficulties that commercial navigation faces on many rivers D. To help explain how point bars are formed