Which one of the following is the correct order of ionic radii?

• A
  $Pr^{3+} > Gd^{3+} > Tm^{3+}$
• B
  $Pr^{3+} < Gd^{3+} < Tm^{3+}$
• C
  $Pr^{3+} > Tm^{3+} > Gd^{3+}$
• D
  $Pr^{3+} < Tm^{3+} < Gd^{3+}$