The two girls had been talking in________low voice all________time during the meeting,which upset the manager.[ ]

A．a；/  

B．/；/  

C．/；the  

D．a；the

由于柴油发动机扭矩曲线比较平缓，在汽车行驶阻力增加时车速下降较快，因此换挡时机应适当提前。