诊断急性心包炎最具特征的体征是（）
A.心界向两侧扩大
B.奇脉
C.心音减弱
D.心包摩擦音
E.Kussmaul征
参考答案：D
人们把高于20000Hz的声音叫 ，即超过了人类听觉上限；把低于20Hz的声音叫 即低于人类听觉的下限。
This doubt had been in my mind for some time, but the research still went on _____ some reasons.[ ]
A. to
B. about
C. for
D. in
