Galvani discovered in the 1700s that frog's legs jerked when touched with a piece of copper and a piece of iron, simultaneously. This happens because action potentials are triggered due to which of the following properties of the metals:
a. Metals conduct charge away from cell membranes
b. Soluble metal ions mimic neurotransmitters
c. The temperature difference between metals and tissues
d. The electro chemical potential difference between the two metals
