chapter 4 answers

This terminology is a little misleading, given that `abstract` usually refers to an incomplete implementation of a contract, and `concrete` often refers to a complete implementation of a contract (where contract refers to the externally accessible operations/members of a type).

An object is an instantiation of a class, which is to say an allocated chunk of memory containing the instance member variables and a reference to the methods (for dynamic dispatch) _defined_ by the class.