A、Learn the local customs. B、Make hotel reservations. C、Book tickets well in advance. D、Have the right documents. D文中提到，在准备出国旅游时

Wherever you go and for whatever reason, it’s important to be safe.(19)While the majority of people you will meet when travelling are sure to be friendly and welcoming, there are dangers—theft being the most common.
    Just as in your home country, do not expect everyone you meet to be friendly and helpful. It’s important to prepare for your trip in advance and to take precautions while you are travelling.(20)As you prepare for your trip, make sure you have the right paperwork. You don’t want to get to your destination only to find you have the wrong visa, or worse, that your passport isn’t valid any more. Also, make sure you travel with proper medical insurance, so that if you are sick or injured during your travels, you will be able to get treatment. If you want to drive while you are abroad, make sure you have an international driver’s license.
     (21)When you get to your destination, use official transport. Always go to bus and taxi stands. Don’t accept rides from strangers who offer you a lift. If there is no meter in the taxi, agree on a price before you get in. If you prefer to stay in cheap hotels while travelling, make sure you can lock the door of your room from the inside. Finally, remember to smile. It’s the friendliest and most sincere form of communication, and is sure to be understood in any part of the world!
19. What is mentioned as a most common danger when people go travelling abroad?
20. What is the most important thing to do when you prepare for your trip abroad?
21. What does the speaker suggest you do when you arrive at your destination?