GPOTony - I understand, but the issue of long, longer, and longest is not in question. You are correct, that you use the comparative form when comparing two objects and the superlative when comparing three or more. However, this only applies
if both or all objects are being compared using the same metric. The metric of 'shortness', if you will, is not, from a
grammatical perspective, the same as the metric of 'longness'. It is true that they are both distance metrics, but grammatically speaking, they are not the same, therefore the comparative rule for two objects doesn't necessarily apply.
You can have a long route and a longer route. (Same metric)
You can have a long route and a short route. (Different metric)
Both of those statements are grammatically correct.
However, "You can have a long route and a longest route" does require at least one intermediate (longer route) to be grammatically correct.
The question, as written, provides no insight as to how the other route is being characterised. To say that all three answers are grammatically incorrect assumes that the other route is judged on the basis of 'longness', and I do not believe that to be a valid assumption.
Comparisons, at least with two objects, can be tricky when you have an 'opposite metric' that can be used -- long vs. short, good vs. bad, light vs. dark, or light vs. heavy, etc.
You can have a good choice and a better choice. (Same metric)
You can have a good choice and a bad choice. (Different metric)
You can have a light box or a lighter box. (Same metric)
You can have a light box or a heavy box. (Different metric)
You can have a light beer, or a dark beer, or any beer will do just fine.
Good Luck
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