题目

smoking  stop  unhealthy allowed  badlystarted  easily  living  thought namedTang Ling often smoked cigarettes(香烟) in his school’s washing room. He  1  smoking at the age of 14. One day he learnt that the NBA star Yao Ming, an ambassador(大使) to the Chinese Association on Tobacco Control (中国控烟协会), asked people not to smoke. Yao Ming was Tang’s favorite star, and his words meant a lot to him. He decided to give up  2  .According to the Chinese Association on Tobacco Control, about 350 million Chinese smoke.  Among them, 50 million are teenagers. Sun Jiangpin在左图所示的电路中，电源电压为18伏保持不变，电阻R1的阻值为10欧。闭合电键S，电流表A的示数右图（a）所示。 ① 求电阻R1两端的电压U1。 ② 求此时滑动变阻器R2连入电路的阻值。 ③ 现有标有“20Ω  2A”、“50Ω  1A”字样的滑动变阻器可供选择，有一个表盘右图（b）所示的电压表可接入电路。当选用标有         字样的变阻器替换R2，并把电压表接入         两点间时（选填“AB”、“CD”、“AB或CD”），在移动电阻器滑片P的过程中电压表示数的变化量△U最大。求电压表示数的最大变化量△U最大。