Explain the use of virtual, sealed, override, and abstract.

Abstract: The keyword can be applied for a class or method.
1. Class: If we use abstract keyword for a class it makes the
class an abstract class, which means it cant be instantiated. Though
it is not nessacary to make all the method within the abstract class to be virtual. ie, Abstract class can have concrete methods
2. Method: If we make a method as abstract, we dont need to provide implementation
of the method in the class but the derived class need to implement/override this method.
Sealed: It can be applied on a class and methods. It stops the type from further derivation i.e no one can derive class
from a sealed class,ie A sealed class cannot be inherited.A sealed class cannot be a abstract class.A compile time error is thrown if you try to specify sealed class as a base class.