Answer 1

https://code.sololearn.com/cuvj6KoXaOl9/?ref=app

A concrete class is a class that has an implementation for all of its methods that were inherited from abstract or implemented via interfaces.
It also does not define any abstract methods of its own. This means that an instance of the class can be created/allocated with the new keyword without having to implement any methods first. Therefore it can be inferred that any class that is not an abstract class or interface is a concrete class.

In my code above, C will be a concrete class as it implements all abstract methods inherited from A and implemented from B. Also, it does not define any abstract methods of its own.