问题
选择题
__________ his work was still as it had been?
A．It was why that
B．Why is that
C．It is why that
D．Why was it that
答案
答案：D
题目分析：句意：为何他的工作仍然与原来一样？分析：Why was it that his work was still as it had been? 其中it为形式主语，真正的主语是that his work was still as it had been。故选D
圆弧导轨，O、P连线水平，M、N与E、F在同一水平高度，水平和圆弧导轨电阻不计，在其上端连有一阻值为R=8W的电阻，在PQ左侧有处于竖直向上的有界匀强磁场，磁感应强度大小为B0=6T。现有一根长度稍大于L、质量为m=0.2kg、电阻为r=2W的金属棒从轨道的顶端P处由静止开始下滑，到达轨道底端MN时对轨道的压力为2mg，取g=10m/s2，求：