Which one of the following pairs represents isobars?

• A
  $_{2}^{3}He$ and $_{2}^{4}He$
• B
  $_{12}^{24}Mg$ and $_{12}^{25}Mg$
• C
  $_{19}^{40}K$ and $_{20}^{40}Ca$
• D
  $_{19}^{39}K$ and $_{19}^{40}K$