Answer 1

-An abstract method is a method that has no implementation (has no body), and it is required for the subclass to implement it.
-A class must be declared abstract if it contains one or more abstract method.
-Abstract classes can't be instantiated, (but its possible with a non-abstract subclass)

Quick example:

abstract class Person {
public abstract string Description(); //No body
}

class Teacher : Person {
public override string Description () {
return "I am a teacher";
}
}

class MainClass {
static void Main() {
var person = new Person() //Error
var teacher = new Teacher() //Ok
}
}