What is a rate determining step?
Let's put this in another context.....
You go shopping on Saturday morning; (i) you park your motor; (ii)you go to the supermarket; you load your trolley; (iii) you wait in line at the cash register; (iv) you buy your groceries; (v) you return to your motor with your purchases.....What is the rate-determining step? I think it is pretty clear that (iii) is the rate determining step, as you will probably spend the longest amount of time waiting in the line with the other punters.
And likewise in chemical kinetics, the rate-determining step is THE SLOWEST STEP in a series of chemical reactions.....
There should be many other answers here that deal with this problem.