根据对话内容，用恰当的单词、短语或句子补全对话。

A: Peter, how about playing basketball together this afternoon?

B: I'm busy. Maybe another time.

A: What are you doing?

B: I'm. I must finish it on time.

A: What plan? Is it very important?

B: Yes. It's about .

A: What kind of volunteer work do you do?

B: I help out at the old people's home and I organize some activities.

A: Can I join you?

B: