A:增大两板间电势差U2
B:尽可能使板长L短一些
C:使加速电压U1升高一些
D:尽可能使板距d小一些
Tom was so careless that he his right arm when he was riding to school
yesterday.
下列操作中,能鉴别空气、氧气和氮气三瓶气体的是 ( )