Two toy cars a and b fixed with spring at front collide as shown in the figure below. Car a has a mass of $$200\,\mathrm{g}$$ and is initially moving to the right. Car b has a mass of $$300\,\mathrm{g}$$ and is initially at rest. When the separation between the cars is minimum,

A

car b is at rest

B

car a has come to rest

C

both cars have the same kinetic energy

D

the K.E of the system is at a minimum