题目

Superman, Spider-Man, Batman and Iron Man …There is no shortage of superheroes. You find them in comic books, on the TV and the big screen. In a survey of fans by the sci-fi and fantasy website, SFX.co.uk, Batman, 70 years old this year, was voted Britain’s favorite superhero. Unlike many crime-fighting superheroes Batman has no special powers. He can't fly like Superman or shoot a sticky web like Spider-Man. Bruce Wayne is born to a wealthy Gotham City business family. However, when his parents are murdered his life changes completely. Wayne feels anger at their deaths and guilty tha如右图所示，N匝矩形导线框以角速度ω在磁感应强度为B的匀强磁场中绕轴OO′匀速转动，线框面积为S，线框的电阻、电感均不计，外电路接有电阻R、理想电流表和二极管D。二极管D具有单向导电性，即正向电阻为零，反向电阻无穷大。下列说法正确的是                                                    （     ）A．图示位置电流为0B．R两端电压的有效值C．交流电流表的示数D．一个周期内通过R的电荷量