A、We may forget what we have thought about. B、Our thoughts may be disturbed. C、Our mind might be harmed. D、We may have difficult

The sense of sound is one of our most important means of knowing what is going on around us. Sound has a waste product, too, in the form of noise. Noise is growing and it may get much worse before it gets any better.
    Scientists have been studying how noise affects people and animals. They are surprised by what they have learned. Peace and quiet are becoming harder to find.
    Noise pollution is a threat that should be looked at carefully.
    There is a saying that it is so noisy that you can’t hear yourself think. Doctors who study noise believe that we must sometimes hear ourselves think. If we don’t, we may have headaches, other aches and pains, or even worse mental problems.
    Ways of making less noise are now being tested. There are even laws controlling noise. We cannot return to the "good old days" of peace and quiet. But we can reduce noise, if we shout loudly enough about it.
19. Why are scientists surprised by the findings in their noise study?
20. What may happen if we cannot hear ourselves think?
21. Which of the following is true according to the passage?