A $0.20\,\text{kg}$ mass hangs from the lower end of a light spring. A further $0.10\,\text{kg}$ mass is attached to the first mass by a thread. The system is left to reach static equilibrium and then the thread is burnt through. At that moment, determine the upward acceleration of the $0.20\,\text{kg}$ mass. (Take $g = 10\,\text{m s}^{-2}$.)
- A$5.0\,\text{m s}^{-2}$
- B$6.7\,\text{m s}^{-2}$
- C$10\,\text{m s}^{-2}$
- D$15\,\text{m s}^{-2}$