A spring – mass system is hanging from the ceiling of an elevator in equilibrium. The elevator suddenly starts accelerating upwards with acceleration $5\mathrm{m}/{\mathrm{s}}^2.$  If mass of the block is 1 kg and force constant of the spring is 1000 N/m, the amplitude of oscillation is 

• A

  10 mm

• B

  15 mm

• C

  5 cm 

• D

  5 mm