It can be inferred from the passage that a clear view of the sun's outer layer is usually vented by_______.

A．how the sun evolved
B．the structure of the sun
C．why scientists study the sun
D．the distance of the sun from the planets

A．By producing pressure waves going in the opposite direction.
B．By mixing high frequency sound waves with low frequency sound waves.
C．By mixing new sound waves with the noise and sending them out together.
D．By making copies of the unwanted sound waves and letting them out a little later.