How would you determine the RDS from the rate and order of a reaction I know what a rate determining step is, and also have knowledge about zero order and first order reactions. But, in one of my tests, I was unable to solve the following question:


  A hypothetical reaction A2 + B2 --> 2AB has a rate law, Rate = k[A2]1 [B2]0 and mechanism of reaction is 
  
  Step-I: B2 --> 2B
  
  Step-II: A2 --> 2A
  
  Step-III: 2A + 2B --> 2AB
  
  The correct statement is
  
  A) Step-I of mechanism is rate determining step of the reaction
  
  B) Step-II of mechanism is rate determining step of the reaction
  
  C) Step-III of mechanism is rate determining step of the reaction if step I and II are reversible
  
  D) Step-I and Step-III both are rate determining step of the reaction if step II are reversible


The correct answer is option B, but how would you approach the problem and solve it?

EDIT: What if rate was given as 

1) Rate = k[A2]1 [B2]1

2) Rate = k[A2]2 [B2]1
The reaction is 1st-order in A2, so for starters we look for a step with A2 on the left, with a coefficient of 1. In fact, only step II has A2 on the left at all, so really we don't have to look any further. 

Comments which may (or may not) make it clearer:

The rate equation says that greater the A2 concentration, the faster the reaction goes, in a linear relationship, and the rate doesn't depend on [B2] at all, since [B2] has an exponent of 0, and anything to the zeroth power is 1. (I suspect that was just put there to make it hard.)

If the whole reaction can be speeded up by adding A2 alone, [A2] must be rate-determining; i.e., [A2] determines the rate, and A2 reacts only in step II.