Section A

A．Too slow for a difficult book though just right for a non-serious one.
B．Too slow for a non-serious book but too hast for a difficult one.
C．Too fast for difficult material though just right for a non-serious book.
D．Too fast for a non-serious book as well as a difficult one.

A．Those with frequencies lower than 20 hertz.
B．Those with frequencies about 21,000 hertz.
C．Those with frequencies lower than 20,000 hertz.
D．Those with frequencies higher than 20,000 hertz.