Which of the equations below represents the correct BALANCED equation?
a. Mg (s) + HCl (aq) --> MgCl2 (aq) + H2 (g)

b. Mg (s) + 2HCl (aq) --> MgCl2 (aq) + H2 (g)

c. 2Mg (s) + HCl (aq) --> MgCl2 (aq) + H2 (g)

d. 2Mg (s) + 2HCl (aq) --> MgCl2 (aq) + H2 (g)