杠杆的平衡条件是 ，动力臂大于阻力臂的杠杆是 杠杆（填“省力”或“费力”）
动力×动力臂=阻力×阻力臂；省力
Hardly _______ to speak when the audience interrupted him.
A．he began
B．he had begun
C．had he begun
D．did he begin
种子的安全含水量高于气干含水量的树种是：（）。
A．云南松
B．桉类
C．油茶
D．柏类
