An observer A is standing stationary on the ground, while another observer B is running at a constant speed of 2 m/s strictly along the positive x-axis. A solid block is sliding along the positive x-axis at 2 m/s with respect to the ground. What is the kinetic energy of the block as calculated purely in the reference frame of observer B?
(A) 21m(2)2 J
(B) 21m(4)2 J
(C) Zero
(D) Indeterminate without the mass of observer B