A、Preparing for dinner. B、Watching TV. C、Cooking tomatoes. D、Washing dishes. A推断题。女士见Robbie回来得早，就让他帮忙准备晚饭(help…with dinner)，由此可知

M: Hi, Mom, I’m home.
W: Hi, Robbie. You’re home from school early. Is there anything?
M: Yeah,[19]they’re getting the assembly hall ready for the graduation ceremony, so we all got to go home early. It’s too noisy for us to study.
W: Well, now that you’re here,[20]you can help me with dinner. I need those potatoes peeled and sliced.
M: Mom, give me a break. Alexandra’s coming over to help me study for my math final.
W: Well, in that case, you can wash the dishes and clean up after dinner.
M: Can I invite Alexandra to stay for dinner? She likes your dishes very much.
W: Of course.
M: Thanks, Mom.
W: You and Alexandra have become good friends, haven’t you?
M: Yes, I like her. She’s a terrific person. I’m going to miss her when she goes back to Greece.
W: Me, too. But we can travel to Greece and maybe we will see her again at that time. Oh, I almost forgot. Would you like to give her a little farewell party?
M: Mom, that would be terrific! Maybe we could make it a surprise.
W: Oh, I don’t know. Surprise parties don’t always work out. It’s time for you to make a decision.
M: Well, let me see. Oh, I have an idea. We could tell her it’s a graduation party for me. When Alexandra arrives, we’ll surprise her.
W: Well, I suppose that might work.
19. Why does Robbie go back home today earlier than usual?
20. What is the woman doing when Robbie arrives home?
21. What does the woman suggest the man do for Alexandra?