If the nuclear forces between two protons,two neutrons,and a proton-neutron are denoted by $F_{pp}$,$F_{nn}$,and $F_{pn}$ respectively,then:

• A
  $F_{pp} \approx F_{nn} \approx F_{pn}$
• B
  $F_{pp} \neq F_{nn}$ and $F_{pp} = F_{nn}$
• C
  $F_{pp} = F_{nn} = F_{pn}$
• D
  $F_{pp} \neq F_{nn} \neq F_{pn}$