The following (I-IV) are the main steps of chemosynthetic ATP synthesis in the light reaction. Arrange them in correct order I.${H^ + }$ concentration gradient established II. ${H^ + }$ diffuses through ATP synthetase III. Carriers use energy from electrons to move ${H^ + }$ across the membrane IV. Electrons from PS-II pass along electron transport chain V. Light excites electrons in PS-II VI. Energy of ${H^ + }$ flow is used by ATP synthetase to make ATP