A.20
B.50
C.30
D.25

A.24 dB.
B.4 dB.
C.6 dB.
D.26 dB.

A.\({v_{3\max }} = 5c{\rm{ }}m/s.\)
B.\({v_{3\max }} = 10cm/s.\)
C.\({v_{3\max }} = 9{\rm{ c}}m/s.\)
D.\({v_{3\max }} = 4{\rm{ c}}m/s.\)

A.language
B.luggage
C.heritage
D.teenage

A.passionate
B.integrate
C.communicate
D.demonstrate

A.conclude
B.conceal
C.concentrate
D.conduct

A.ethnically
B.ethics
C.ethnicity
D.ethnic

A.humor
B.responsibility
C.self
D.honesty

A.avoidable
B.unavoidable
C.stoppable
D.unstoppable

A.at once
B.once
C.one time
D.for once