Use of definite articles and plural in lists with common object

I want to write the following sentence in a compact way:

"The difference between the old scheme and the new scheme lies in..."

Which one of the following is correct?

1. The difference between the old and new scheme lies in...


2. The difference between the old and new schemes lies in...


3. The difference between the old and the new scheme lies in...


4. The difference between the old and the new schemes lies in...

Option 1 sounds more natural to me but I cannot explain why. More important, English is not my mother tongue. In other words, I would like to know:

• Is it necessary to repeat the definite article for each element of the list?


• Does the common object (in this case, the scheme) become plural when it is listed multiple times with different pre-modifiers (in this case, old and new)?