whats the use of all of this?

Question

Hello,
I have multiple questions

1:

as example

class A {}
class B : A {}

class main
{
//i know to initiate a new initiate like this
A hi1 = new A();

//but whats the use of doing it like this: //and why should I do it this way
A hi2 = new B(); //why is the second thing B ???
}

2:
whats the diverence between virtual, abstract and Interface // how I understand it they all make the same thing

thank for anwers

Answer 1

This may answer your first question :
https://www.sololearn.com/discuss/2092037/?ref=app