A \(500 ~\text{kg}\) object slides across a horizontal surface where the coefficient of kinetic friction is \(\mu=0.7.\) To maintain a constant velocity of \(10~\text{m/s},\) an external force is applied horizontally in the direction of motion. The object travels a total distance of \(4~\text{km}\) under these conditions. The work done by the external force is:
 1. \(3.5 \times 10^6~\text{J} \)
2. \(28 \times 10^6~\text{J} \)
3. \(7 \times 10^6 ~\text{J} \)
4. \(14 \times 10^6 ~\text{J} \)