Given the masses of various atomic particles $m_{p} = 1.0072 \ u$,$m_{n} = 1.0087 \ u$,$m_{e} = 0.000548 \ u$,$m_{\bar{v}} = 0$,and $m_{d} = 2.0141 \ u$,where $p \equiv$ proton,$n \equiv$ neutron,$e \equiv$ electron,$\bar{v} \equiv$ antineutrino,and $d \equiv$ deuteron. Which of the following processes is allowed by momentum and energy conservation?

• A
  $n + p \rightarrow d + \gamma$
• B
  $e^{+} + e^{-} \rightarrow \gamma$
• C
  $n + n \rightarrow$ deuterium atom (electron bound to the nucleus)
• D
  $p \rightarrow n + e^{+} + \bar{v}$