Task: Question Answering
Input (English): To keep a heavy box sliding across a carpeted floor at constant speed, a person must continually exert a force on the box. This force is used primarily to overcome which of the following forces?  
 (A) Air resistance (B) The weight of the box (C) The frictional force exerted by the floor on the box (D) The gravitational force exerted by the Earth on the box
Output (English): C