What do they mean when they say id attribute is unique and can only be used once?

Question

*To make the question clearer
Distinguish the one which is correct :
And please explain why the other one is wrong!
https://code.sololearn.com/WubzbGHUgZ3Z/?ref=app
https://code.sololearn.com/WiXzc7200eZh/?ref=app

Answer 1

choice A is wrong, use class instead
choice B is correct

Answer 2

to see the difference, try use this CSS syntax

use this CSS syntax in choice A:
#paragraph1 {
color: red;
}

use this CSS syntax in choice B:
#paragraph1 {
color: red;
}
#paragraph2 {
color: red;
}

Answer 3

a simple explaining

id must unique:
think if id is yourself in this world, only one and different from others.

id can only be used once:
think if id is a number, because number only seen once if you write the numbers from 0 to infinity,

Answer 4

If id is unique and can't be used more than once in a document why do we use it then, why not just use class???