For the reaction SO₂ + O2 → SO₃, which of the following relationships is correct at constant temperature?
1) The rate of formation of SO₃ is directly proportional to the concentration of SO₂
2) The rate of formation of SO₃ is directly proportional to the concentration of O2
3) The rate of formation of SO₃ is directly proportional to the concentration of both SO₂ and O2
4) The rate of formation of SO₃ is inversely proportional to the concentration of both SO₂ and O2