小明在网络中看到镁是航空工业的重要材料，欲通过实验探究镁的性质．

如图所示，电源电压不变，R₁=20Ω，只闭合开关S时电流表的示数是0.15A，再闭合开关S₁时电流表示数是0.21A．求：

下列酒精灯的使用方法正确的是（　　）

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