Identify the correct decreasing order of ionic radii of the following lanthanoids: $Ce^{3+}$,$Pm^{3+}$,$Sm^{3+}$,and $Gd^{3+}$.

• A
  $Sm^{3+} > Gd^{3+} > Ce^{3+} > Pm^{3+}$
• B
  $Gd^{3+} > Pm^{3+} > Ce^{3+} > Sm^{3+}$
• C
  $Ce^{3+} > Pm^{3+} > Sm^{3+} > Gd^{3+}$
• D
  $Pm^{3+} > Sm^{3+} > Ce^{3+} > Gd^{3+}$