Can we interchange the #and. | For id and class

Question

#intro {
Color:red;
Background-color:green;
}
.Test {
Color:green;
Background-color:red;
}
<div id="intro">... <div>
<div class="Test">... <div>

Answer 1

You can interchange it..
as long as you follow the following rules:

Each id can only occur once.
Whereas class can occur one or many times.

Answer 2

use both.
when you know the style going to be reused then use class, otherwise use id.

Answer 3

Thank you calvin

Answer 4

Thank you