请结合上下文将下列对话补充完整。

1. — What did he do yesterday morning?

    — He _______________ breakfast at home.

2. — When did you go to school?

    — I _______________ to school at 7: 30.

3. — Where did Lily sing and dance last Sunday?

    — She _______________ in the music room.

4. — Did your father take pictures on his holiday?

    — No, he_______________. He _______________ mountains.

瀑布模型存在的问题是( )。

A．用户容易参与开发

B．缺乏灵活性

C．用户与开发者易沟通

D．适用可变需求