司机驾驶汽车在平直公路上匀速行驶，突然遇到紧急情况刹车直到停止运动，从司机发现情况到停止运动这段时间内汽车的速度—时间图象如图所示，则下列说法正确的是

A．司机的反应时间是t      

B. 司机的反应时间是9t

C．汽车刹车的加速度大小是

D汽车刹车的加速度大小是

A vessel aground may have negative GM since the（）.

A.decrease in KM is equal to the loss of draft

B.virtual rise of G is directly proportional to the remaining draft

C.lost buoyancy method is used to calculate KM, and KB is reduced

D.displa ement lost acts at the point where the ship is aground