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).

d. Both (a) and (b).