$\alpha$-decay of a parent nucleus $\mathrm{X}$ results in a daughter nucleus Y. If $m_x, m_y$ and $m_a$ are the masses of the parent nucleus, the daughter nucleus and the $\alpha$ - particle respectively, then the net kinetic.energy gained in the process is

• A

  $\left(m_x+m_a-m_y\right) c^2$
• Correct answer
  B

  $\left(m_x-m_y-m_a\right) c^2$
• C

  $\left(m_x+m_y+m_a\right) c^2$
• D

  $\left(m_x+m_y-m_a\right) c^2$

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