三相交流电机在三相交流电通入电机定子绕组后，在空间产生（）。
A、电压
B、磁场
C、电流
D、旋转磁场
参考答案：D
Ever since the Greens moved to the country a year ago, they________better health.[ ]
A. are enjoying
B. have been enjoying
C. could have enjoyed
D. had enjoyed
如（x+m）与（x+3）的乘积中不含x的一次项，则m的值为（ ）
A．-3
B．3
C．0
D．1
