The wave function $R_{2,0}$ for hydrogen atom is $R_{2,0} = K(2 - r/a_0) \exp(-r/2a_0)$,where $K$ is a constant. The distance $r$ at which $R_{2,0}$ exhibits a nodal point is at:

• A
  $a_0$
• B
  $2a_0$
• C
  $3a_0$
• D
  $4a_0$