A block of mass m is attached to two identical light springs of force constant k, one on each side, on a horizontal friction-free surface. The springs are fixed to rigid supports at their far ends. If the block is pulled a small distance x to the right of equilibrium and released, the net restoring force on the block at that instant is
a) 0 (no net force)
b) kx directed to the left
c) 2kx directed to the left
d) 2kx directed to the right