以下说法正确的是（　　）

A．短跑运动员用11.9s跑完了100m．“11.9s”是时间

B．一节课是45分钟．“45分钟”是时刻

C．当研究一列火车全部通过桥所需的时间时，火车不能被看做质点

D．速率是标量

Which is the proper method of determining whether a portable CO₂ fire extinguisher needs recharging?（）

A.Check the tag to see when the extinguisher was last charged

B.Release a small amount of CO₂; if the CO₂ discharges，the extinguisher is acceptable

C.Weigh the extinguisher and compare the weight against that stamped on the valve

D.Recharge the extinguisher at least once each year