For this question:
Considering the following sequence diagram, it is CORRECT to state that:
Select one or more:
a. Message 4 might never be executed.
b. Message 5 is not complete until the Customer receives an message confirming creation of the ShoppingCart.
c. Message 7 corresponds to behaviour belonging to the ShoppingCart class.
d. Message 6 is an example of a timed event which can only take place after other messages have occurred.
e. Book and Bookshop are objects.
The correct options are A and C. But I believe the correct option is A and E. E should be correct because Bookshop and Book are clearly objects since they are enclosed in rectangular shapes. Why are options A and C correct and not A and E?