Any concrete subclass that extends class Foo:

Consider the abstract class 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();
}
```

a) Must implement a method called calculate.
b) Will not be able to access the instance variable a.
c) Will not be able to instantiate an object of class Foo.
d) All of the above.

d) All of the above.