红外线心率计门控电路的作用是输出（）
A.30s的高电平
B.30s的低电平
C.60s的低电平
参考答案：B
There was a _________ look on her face so I repeated the question.
A．puzzled
B．puzzling
C．puzzle
D．puzzlement
热应力、热冲击、热疲劳的概念。
