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)