A、He has a big appetite. B、He eats only at regular mealtimes. C、He avoids new foods. D、He eats only healthful foods. A选项中的He，big

W: Hi, Jack. Tomorrow’s the big day! You must be excited about going to France for your first international conference.
M: The trip, yes, but not the preparation.
W: What do you need to do except pack?
M: You know, you always feel so awful whenever you make a big change in time zones. This time I’m trying an anti-jet-lag program so I’ll be alert even for the first day’s lectures.
W: But how can you avoid jet lag? With that late flight and a six-hour difference in time, it’s bound to take you a couple of days to adjust.
M: Well, this program’s supposed to get your body to feel like it’s in the new time zone before you leave. The theory is that the food you eat actually tells your body when to be active and when to be restful, so changing your diet can help you be ready for the time change. You have to alternate feasting and fasting...you know, sometimes eat a lot, sometimes just a little.
W: OK. Now I see why you’re not enjoying the preparation. I can’t quite picture you fasting!
M: Actually the worst is already over. Two days before the flight, you’re supposed to have only light meals and limit yourself to 800 calories.
W: That must have been tough.
M: Yeah, but I’m making up for it today. Feast day is much better...though I still don’t get to eat any snacks after dinner.
Questions 19 to 22 are based on the conversation you have just heard.
19. What is the conversation mainly about?
20. How is the program that Jack is using supposed to work?
21. Why isn’t Jack enjoying the preparation for his trip?
22. What can be inferred about Jack’s usual eating habits?