[مستر وليد السيد]

Sender Atef Ahmedteacher

I don't know why I tend to using the superlative in this sentence. Maybe because of the article "the" or because the preposition "of". So I think it sounds more normal if we say: Nora is the shortest of the two girls. Anyhow I will try to find proof for my point of view.
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Editor's reply:
It has nothing to do with "the" or "of". This simply teling us that there are two girls, Nora is one of them.
As I said before, many people do use the superlative here, but others believe it is wrong to use the superlative when only two things are being compared. I don't know what the people who write and grade the exams think on this issue, so I think it is safer to be conservative here. Use the comparative when you are comparing only two things/people or when you are comparing one thing/person to a group.
Nora is the taller of the two girls.
Nora is taller than Samia.
Nora is taller than the other girls in her class.|
Nora is the tallest girl in her class.