A horizontal force drags a box across a level surface. The box gains $30\,\text{J}$ of kinetic energy and friction between the box and the surface generates $10\,\text{J}$ of thermal energy. How much work is done by the force?
- A$10\,\text{J}$
- B$20\,\text{J}$
- C$30\,\text{J}$
- D$40\,\text{J}$