激动时使血管平滑肌收缩的是
A．a受体B．β1受体C．β2受体D．M受体E．N2受体
参考答案：A
Tom kept quiet about the accident _______lose his job.
A．not in order to
B．so as not to
C．in order to not
D．not so as to
核心部分数据交换方式有（）。
A.电路交换
B.报文交换
C.分组交换
