题目

—Are you familiar with the music?— Yes. There was a time _____ this kind of music was quite popular. A . that B . when C . with which D . about which 答案：B春季多风，小明自制了一种测定风速的装置，如图所示.其中风速表是由电压表改装而成，R为定值电阻，R1为滑动变阻器，“T”形管道的竖直管内装有可上下自由移动的轻质活塞A，A和滑动变阻器的滑片相连.请分析说明该装置测量风速的原理.(提示：U=IR)