Classically, the kinetic energy of a particle will quadruple when the speed is doubled. For example, if a particle has KE = 20 J when it moves at speed v, the particle will have KE = 80 J if it has speed 2v.

But the relativistic expression behaves differently. Let's say that a particle has KE = 90 J when it moves at speed c/2. If we check the relativistic KE at a doubled speed, what are we going to get?

A). It is at infinity. Because the cost in energy to have mass moving at the speed of light is infinite.

b). At zero.