Consider the Abstract Superclass Below

Consider the abstract superclass below:
Public abstract class Foo
{
Private int a;
Public int b;
Public Foo(int aVal, int bVal)
{
A = aVal;
B = bVal;
}
Public abstract int calculate() ;
}
Any concrete subclass that extends class Foo:

A) Must implement a method called calculate.
B) Will not be able to access the instance variable A.
C) Neither (a) nor (b) .
D) Both (a) and (b) .