单项选择题


Questions 17 to 20 are based on the following passage. At the end of the passage, you will he given 20 seconds to answer the questions.
Now listen to the passage.
Why does the speaker give the example of a hub of a wheel surrounded by spokes

A.To explain how the starfish reproduces.
B.To illustrate the shape of the starfish.
C.To demonstrate the defense mechanisms of starfish.
D.To show how starfish moving.