The idea of using A():base() is to give some parameter to the base class to be initialized by the constructor of the base class. In this example there is no need of calling the base class constructor cuz you don't have any parameters:)
For example you can
public B(int a,int b):base(b)
{
}
and to call
B obj=new B(c,d);
here your parameter d will be initialized by the constructor of the base class(A)
And btw be careful how you inherit classes . In the common case you inherit like this
class B:public A
It can be private and protected too and it will change the access levels for member variables from the base class