When people exercise for long periods of time, their muscles require increased amounts of oxygen. Which system is responsible for carrying this oxygen to the muscles?.

First definition:

Duru's definition: A line segment is a line connecting two endpoints

                                               ⇅

Teacher's comment: What you've written is true, but it isn't a correct definition. A ray would also fit your defenition

Second definition:

Oliver's definition: A line segment is when three points are all on the same line.

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Teacher's comment: Were you thinking of the team collinear?

Third definition:

Carlos' definition: A line segment is part of a line, not the whole thing.

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Teacher's comment: I think you have the right idea, but you need to be more careful with your language.

Explanation:

A line segment is part of a line that connects points. It limited consequently, it has length.

Duru's definition: A line segment is a line connecting two endpoints

                                               

It is missing that it is just a part of the line. A ray also connects two points but extends infinetly in one direction. Thus,  Duru's definition is not so specific.

That justifies teacher's comment:

Teacher's comment: What you've written is true, but it isn't a correct definition. A ray would also fit your defenition

Second definition:

Oliver's definition: A line segment is when three points are all on the same line.

 

The constraint of a line segment to three points all on the same line corresponds to collinear points. Two points always form a line,, but three points lie on a line only in they are collinear.

That justifies, theacher's comment:                          

Teacher's comment: Were you thinking of the team collinear?

Third definition:

Carlos' definition: A line segment is part of a line, not the whole thing.

Mathematicall language is formal. You must use precise words. An expression as 'whole thing" is not formal.

That justifies teacher's comment:

Teacher's comment: I think you have the right idea, but you need to be more careful with your language.