An assembly of identical spring-mass systems is placed on a smooth horizontal surface as shown. At this instant,the springs are relaxed. The left mass is displaced to the left and the right mass is displaced to the right by the same distance and released. The resulting collision is elastic. The time period of the oscillations of the system is

• A
  $2 \pi \sqrt{\frac{2 m}{k}}$
• B
  $2 \pi \sqrt{\frac{m}{2 k}}$
• C
  $\pi \sqrt{\frac{m}{k}}$
• D
  $2 \pi \sqrt{\frac{m}{k}}$