从方框中选择最佳选项完成对话，有两项多余。
A. Who is the main character in it?
B. It's a good action film.
C. It was fantastic.
D. What kind of film is it?
E. What do you think of it?
F. Where and when shall we meet?
G. You'll have a chance next time.
A: Hi, Daming. Did you go to the concert last night?
B: Yes, I went there with my parents.
A: ____
B: Very wonderful. Some famous singers like Zhu Zhiwen and Li Yuchun sang songs at the concert.
A: What a pity! I didn't go there because I had to look after my sister at home.
B: ____ By the way, would you like to see a film with me tonight?
A: Yes, I'd love to. ____
B: It's an action film.
A: Great! I like action films. What's the name of it?
B: Chinese Zodiac(《十二生肖》). It is my favourite action film.
A: ____
B: Jackie Chan. Maybe this is his last action film.
A: ____
B: What about 7:00 at Hongxing Cinema? The film begins at 7:20.
A: OK, see you then.
B: Bye.
， 若目标函数z=ax+by（a>0，b>0）的最大值为12，则
的最小值为( ).