A spring of force constant k is slowly compressed by a distance x. Which option correctly states the work done (i) by the external agent on the spring and (ii) by the spring force during this process?
a) (i) −21kx2,(ii)+21kx2
b) (i) +kx2,(ii)−kx2
c) (i) +21kx2,(ii)−21kx2
d) (i) +21kx2,(ii)+21kx2