Superlatives or comparatives

[Unregistered]

Last month, I had a problem with my exercise:
Mary is ___________of the two sisters.
A the younger
B the youngest

My teacher sais that the answer was A.
But I foung in one of my Grammar books said that the two is ok.
I took it to my teacher then she said that A is better <the book said "we sometimes use comparatives instead of superlatives">
After a while, I read some kinds of leaflet ,I guess, "KET INformation for Candidates" Of Cambridge.
In that material, I came across one sentence in a reading like this
"But they think the on the fish ate is the best one"
In this situation, there are 2 rings and they use superlatives to compare the 2 things.
So what is the answer? What should I do when I do other similar exercise?

[Teia]

Last month, I had a problem with my exercise:
Mary is ___________of the two sisters.
A the younger
B the youngest

My teacher sais that the answer was A.
But I foung in one of my Grammar books said that the two is ok.
I took it to my teacher then she said that A is better <the book said "we sometimes use comparatives instead of superlatives">
After a while, I read some kinds of leaflet ,I guess, "KET INformation for Candidates" Of Cambridge.
In that material, I came across one sentence in a reading like this
"But they think the on the fish ate is the best one"
In this situation, there are 2 rings and they use superlatives to compare the 2 things.
So what is the answer? What should I do when I do other similar exercise?

Superlative adjectives are used when describing three items or more. Superlative adjectives are never used with two items.
When comparing two items, the comparative is the correct choice. Your teacher was right when s/he said that the correct answer is A.