After putting on a self-contained breathing apparatus，you open the air supply and hear a continuous ringing of a bell. What does this mean?（）.

A.The unit is working properly

B.The face mask is not sealed properly

C.The air bottle needs to be refilled

D.The air supply hose has a leak

抗高血压药的种类及代表药物

β受体阻断剂(β-RB)

A．氯沙坦
B．吲达帕胺
C．依那普利
D．氨氯地平
E．美托洛尔