与鸟类不同，哺乳动物具（）体动脉弓。

B= Bill     F= Bill's friend
F: What's wrong, Bill? You are not looking happy.
B: I'm not. I feel a little worried about my English.
F: 1           
B: I'm not getting along well with it.
F: Why not?
B: Well, I seldom have chances to meet English people.
F: 2           
B: Where should I go?
F: 3           
B: But... it seems English people never speak to me.
F: Ah! 4           
B: 5            
F: Oh, weather! English people are always interested in the weather.