—No. But it has been 3 years since I last here.
A:visit; come
B:visited; came
C:have visited; came
D:have visited; come
当黑龙江还是冰天雪地时,海南岛却是农忙时节,造成这种差异的原因是( )
图甲是测量小灯泡(U额=2.5V)电阻的电路.电源电压恒为4V.
已知直线l1:ax+2y+6=0,l2:x+(a﹣1)y+a2﹣1=0,若l1⊥l2,则a= ____ ,若l1∥l2,则l1与l2的距离为____