12. Which expression does not represent the rate of the following reaction?
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
a. $$\frac{\Delta[Mg]}{\Delta t}$$
b. $$\frac{\Delta[H_2]}{\Delta t}$$
c. $$-\frac{1}{2}\left(\frac{\Delta[HCl]}{\Delta t}\right)$$
d. $$\frac{\Delta[MgCl_2]}{\Delta t}$$