Which of the following statements is false?

• A
  $Ca^{2+}$ ions are not important in maintaining the regular beating of the heart.
• B
  $Mg^{2+}$ ions are important in the green parts of the plants.
• C
  $Mg^{2+}$ ions form a complex with $ATP$.
• D
  $Ca^{2+}$ ions are important in blood clotting.