Question: You have a when expression for all of the subclasses of the class Attribute. To satisfy the when, you must include an else clause. Unfortunately, whenever a new subclass is added, it returns unknown. You would prefer to remove the else clause so the compiler generates an error for unknown subtypes. What is one simple thing you can do to achieve this?

1. Replace `open` with `closed`
2. Replace `open` with `sealed`
3. Replace `open` with `private`
4. Replace `open` with `public`

Answer: The correct answer of the above question is Option B:Replace `open` with `sealed`