Which of the following is a properly balanced equation?

1. 2 C3H12 + 8 O2 --> 5 CO2 + 6H2O

2.Cu + AgNO3 --> Cu(NO3)2 + Ag

3.2 Al + O2 --> Al2O3

4. 3 CO + Fe2O3 --> 2 Fe + 3 CO2