18.
A is a relationship is a relationship between a sub classes and super classes through inheritance.
A relationship between two object where one object refers to the other object and contains the state of the other object.
19. It is a bad design because a square object cannot substitute a rectangle object.
20.
having 52 different sub classes will result in unnecessary complications, it would be easier to define suites and rank as fields to be used for your different card objects.
21. Creating a separate dividend class is smart, since the stock class already function properly so there is no need to mess with it, and not all dividend stock objects can work well with stock objects since not all stocks pay dividends.