题目

 Unfortunately, the research was limited __________ increasing economic profits, _________ protecting the environment.   A. to; other than        B. by; rather than   C. to; rather than    D. by; more than 答案：C如图所示，电源电压U保持不变，当可变电阻R3的滑动端P自左向右滑动的过程中，各电阻两端的电压、通过各电阻的电流分别值怎样变化？