A block of mass m is attached to a horizontal ideal spring with force constant k. The block is pulled a distance x to the right of the equilibrium position and released from rest on a frictionless surface. Which expression gives the block’s instantaneous acceleration a at the moment of release?
(a) a=mkx
(b) a=−2mkx
(c) a=−mkx
(d) a=−m4π2kx