The rate of nitration of toluene in nitromethane with excess of HNO3 is constant and the reaction is zero order with respect to toluene. If the rate is temperature dependent, which of the following can be expected as a rate determining step?

Formation of nitronium ion NO2+ requires some energy and is the slowest step of the reaction. Hence, it is expected as a rate determining step.Protonation is usually fast, so cannot be the rate determining step and formation of arenium ion cannot be the rate determining step because the reaction is independent of the concentration of toluene.