碘是合成甲状腺激素的重要原料，人体必须从外界获取一定量的碘。下图是人体对物质的吸收、利用和排出过程示意图，①~③分别代表某种生理过程，A、B表示某种物质。请回答下列问题。

根据短文理解，完成下了各题。

Dear Tom,

    I go on a trip to Beijing with my father Rick. On the first day, we visit (参观) the Palace Museum. It is in the centre of Beijing. There are many rooms in it. You can see “No photos!” in some places. If so, you need to follow the instructions (指示). On the second day, we visit the Summer Palace. It's great fun to boat there. I can swim, so I'm not afraid of water. Tomorrow we will go to the Great Wall.

I also make a new friend Tony who comes from Australia. When he leaves, he gives me his e-mail address and telephone number. After the six-day trip, we can go back to New York by plane. I hope to see you soon.

Bill