When the mass attached to a spring is increased from 4 kg to 9 kg, the time period of oscillation increases by $0.2 \pi \mathrm{~s}$. Then the spring constant of the spring is

• A

  $80 \mathrm{~N} \mathrm{~m}^{-1}$
• B

  $200 \mathrm{~N} \mathrm{~m}^{-1}$
• C

  $50 \mathrm{~N} \mathrm{~m}^{-1}$
• Correct answer
  D

  $100 \mathrm{~N} \mathrm{~m}^{-1}$

Hence, the correct answer is option: 'D'.