What is or are considered as a tragedy in Germans’ friendship

A．By mixing high frequency sound waves with low frequency sound waves.
B．By producing pressure waves going in the opposite direction.
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.

A．Because she expects him to be well-informed.
B．Because he reads too many books.
C．Because he always wastes his time.
D．Because he doesn’t learn how to separate the important from the unimportant.