Propose the mechanism of a hypothetical reaction

I was given a hypothetical reaction $\ce{A + 2B + 4C -> 2D + E}$ for which I have to propose the possible mechanism with a requirement, that the third step is rate determining.


From the other given inputs I determined that the rate law for this reaction will be $\ce{B + 2C}$.


I came up with a number of "possible" reaction steps, all of which seem to me equally unrealistic. Examples:



```
A +  B -> X
C +  X -> Y 
B + 2C -> 2D + Z
C +  Z +  Y -> E

```

This does not make sense to me as $\ce{C}$ reacts with $\ce{X}$, $\ce{B}$ and with the product of its reaction with $\ce{B}$.



```
A +  B -> X
C +  X -> 2D + Y 
B + 2C -> Z
C +  Z +  Y -> E

```

Same problem here, $\ce{C}$ reacts with just about everything.



```
A +  B -> 2D + X
C +  X -> Y 
B + 2C -> Z
C +  Z +  Y -> E

```

Needless to say I am skipping the variation where the last step uses 3 reactants and produces 2 products, as it's obviously introduces another multi-step reaction:



```
A +  B -> X
C +  X -> Y 
B + 2C -> Z
C +  Z +  Y -> 2D + E

```

Of course there can be 4 variants of each of the above approaches, achieved by simply combining the variables in a different order.


The above considerations do not seem to be bothering the author of the course. For the hypothetical reaction $\ce{2A + 3B + 5C -> 2D}$ they consider the following the correct answer:



```
2A + B -> W
2C + W -> D + X
B + 2C -> Y
C +  X -> Z
B +  Y + Z -> D

```

Here $\ce{C}$ reacts with $\ce{B}$ and the product of its own reaction with $\ce{W}$. I do not understand how it is possible to come up with a "realistic" mechanism of a reaction where the specific substances are unknown.


I tries google and found a number of videos explaining reaction mechanisms on the example of $\ce{NO2 + CO -> NO + CO2}$. In those videos they propose a mechanism, where $\ce{NO2}$ reacts with itself to form $\ce{NO}$ and $\ce{NO3}$. (Are they kidding? If this was possible, production of nitric acid would be a joke.) If I was to follow this approach, there could be a number of variations of the mechanism above where each of the reactants and intermediate products reacted with itself. I estimate that would introduce several hundreds of variants.


How is an average grade 12 student expected to solve this task?


