等响曲线指的是在等响度条件下（）和频率之间的关系曲线。
A、声衰减
B、声压级
C、声功率
D、声能量
参考答案：B
Mr. Smith graduated from college at the age of 23, and ___________ he has been working as a teacher in a middle school.
A．after that
B．from then on
C．from then
D．since then
简述近代小说繁荣发展的原因。
