In the process of balancing the equation:
Al + Cl
2 
AlCl
3
Student I writes: Al + Cl
2 
AlCl
2
Student II writes: Al + Cl
2 + Cl

AlCl
3
Student III writes: 2 Al + 3 Cl
2 
2 AlCl
3
Which student has written the correct balanced chemical equation for the reaction?