A 3.0-kilogram steel block is at rest on a frictionless, horizontal surface. A 1.0-kilogram lump of clay is propelled horizontally at 6.0 meters per second toward the block, as shown in the diagram below.

Which of the following best describes internal energy? a. the difference between the kinetic and potential energies of the particles in a system b. the sum of the kinetic and potential energies of the particles in a system c. the sum of the kinetic and thermal energies of the particles in a system d. the difference between the kinetic and thermal energies of the particles in a system