Questions 17～20 are based on the following passage. You now have 20 seconds to read questions 17～20. How do naturalists regard the rattle of the rattlesnake

A．They believe that the snake is able to control the rattle.
B．They believe that the snake is unaware of its rattle.
C．They believe that the rattle is a nervous action.
D．They believe that the rattle is a deliberate warning.