肉桂中有镇静、镇痛和解热作用的有效成分是
A．桂皮醛
B．醋酸桂皮酯
C．桂皮酸
D．苯甲酸
E．生物碱类
参考答案：A
It is the prevention of disease ______ its successful treatment that has led to the rapid increase of the
world's population. [ ]
A. rather than
B. more than
C. other than
D. less than
从电视接收天线到扬声器，伴音信号频率（幅度）经过哪些变换？
