根据下面对话中的情景，在每个空白处填人适当的语句，使对话恢复完整。

A: Summer holiday is coming next month. What are you going to do?

B: We are going to Xi'an.  1   

A: Oh, your grandparents must be very happy to see you again.

B: Of course they are.

A:   2   

B: About four or five days.

A:   3   

B: No, by train. Taking a bus is a little dangerous.  4   

A: Maybe my parents will take Dick and me to some cities in the south.

B: That'll be great.  5   

A: Maybe Hainan, Shenzhen and Guangdong.

B: Oh, They are all great cities. I think you'll have a good time.

A: You, too.

质量分别为m、2m、3m的物块A、B、C叠放在光滑的水平地面上,现对B施加一水平力F,已知AB间、BC间最大静摩擦力均为f₀,为保证它们能够一起运动,F最大值为（   ）

A．6f₀              B．4f₀                   C．3f₀               D．2f₀