根据上下文补全对话    

Jack: Hello!    1    

Tom: Sure. Wait a minute.

Mary: Hello.    2     

Jack: This is Jack speaking. Mary, my mother is ill.  So I must stay at home to look after her.

Mary:     3     What can I do for you?

Jack: Would you like to tell Miss Gao that I can't go to school this afternoon?

Mary:     4    

Jack: Thanks.

Mary:     5    

1.           2.             3.             4.             5.            

心肺复苏时，心脏按压与人工呼吸之比应为（）。

A.4：1

B.5：1

C.10：2

D.15：2

E.30：2