For which of the following reactions is the $\Delta H^o$ value equal to the first ionization energy of $Ca$?

• A
  $Ca^{+}_{(g)} \to Ca^{2+}_{(g)} + e^-$
• B
  $Ca_{(g)} \to Ca^{+}_{(g)} + e^-$
• C
  $Ca_{(s)} \to Ca^{+}_{(g)} + e^-$
• D
  $Ca_{(g)} \to Ca^{2+}_{(g)} + 2e^-$