（2015•丹东）如图所示，电源电压不变，闭合开关S后，滑动变阻器的滑片P向b端移动的过程中，下列说法正确的是（　　）

随着各地政务大厅建设的推进，集中办公、网上审批、串联审批、留痕监督等，已经成为政务审批中的一种常态化流程和机制。推行政务公开（  ）

①是有效制约和监督权力的关键            

②有助于政府提高依法执政水平

③有利于规范政府及其工作人员的行政行为  

④便于公民监督政府的工作，防止权力滥用

   John lives on a farm. The farm is not very big. His parents grow rice and corn. They don’t use animals to do farm work any more. Today they use a tractor(拖拉机). It works faster and better. After he does his work in the fields, he likes to sit and look at the blue sky and the green hills. He thinks the country is more beautiful than the city. In the city, he can’t hear the animals. In the country, he can hear birds singing. He likes to play with his dog, Cody. Cody is a very interesting dog. He likes to follow people. When they stop, he stops.
   When they walk, he walks. John thinks Cody is the best dog of all.