$O^{2-}, F^-, Na^+, Mg^{2+},$ and $Al^{3+}$ are isoelectronic ions. What do their ionic radii represent?

• A
  Decrease from $O^{2-}$ to $F^-$ and increase from $Na^+$ to $Al^{3+}$.
• B
  Effective decrease from $O^{2-}$ to $Al^{3+}$.
• C
  Effective increase from $O^{2-}$ to $Al^{3+}$.
• D
  Increase from $O^{2-}$ to $F^-$ and then decrease from $Na^+$ to $Al^{3+}$.