A truck moving on a smooth horizontal surface with a uniform speed u is carrying dust. If a mass \[\Delta m\] of the dust leaks from the truck in vertical downward direction in time \[\Delta t,\] the force needed to keep the truck moving at its constant speed is

• A. Zero
• B. \[\frac{u\,\Delta m}{\Delta t}\]
• C. \[\Delta m\times \frac{du}{dt}\]
• D. \[u\frac{\Delta m}{\Delta t}+\Delta m\times \frac{du}{dt}\]