Complete the dialogue with proper words or sentences.

A: Excuse me.     ?

B: The nearest restaurant is about 8 kilometers from here. You'd better take a bus.

A: Can you tell me the way to the bus stop?

B: Turn right at the second crossing. Keep walking and you'll see it on your left.

A:    2  ?

B: About ten minutes' walk.

A: Thank you very much.

C(waiter):    3  ?

A: I'd like Beijing Roast Duck and a bottle of beer.

C: OK. Wait a moment.

(After twenty minutes)

C:    4  ?

A: It is delicious.

C: I'm glad that you like it. Would you like something else?

A:   5   . I'm full. May I have the bill?

C: OK.

在具有通道处理机的系统中，用户进程请求启动外设时，由 (49) 根据I/O要求构造通道程序及通道状态字，并将通道程序保存在 (50) ，然后执行启动“I/O”命令。

A．用户进程
B．应用程序
C．通道
D．操作系统