Consider the MustInherit base classes below:

Public MustInherit Class Foo
Private a As Integer
Public b As Integer

Public Sub New( ByVal aVal As Integer, ByVal bVal As Integer )
a = aVal
b = bVal
End Sub

Public MustOverride Function Calculate() As Integer
End Class ' Foo
Any concrete subclass that inherits class Foo:
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.