条痕为白色，用指甲可以刮下白粉，触之有滑润感的药材是
A．石膏
B．炉甘石
C．滑石
D．浮海石
E．寒水石
参考答案：C
电火花是电极间的击穿放电；大量电火花汇集起来即构成电弧。电火花和电弧是引起火灾的重要原因。电弧的引燃能力很强，通常情况下，电弧的温度最高可达( )℃以上。
A．4 000
B．6000
C．8 000
D．5 000
Between 1897 and 1919 at least 29 motion pictures in which artificial beings were portrayed ______ .
A．had produced
B．have been produced
C．would have produced
D．had been produced
