Why don't you _________your teacher for help?
A:ask
B:to ask
C:asking
D:asks
20+50____70 20____70=90 10____30=40
80-30____50 20____20=0 60____20=40
读图,下列叙述正确的是( )
如图所示,用轻质杆将一电灯吊起,O点是支点.请画出动力F1的力臂L1和阻力F2的示意图.