Refer to the exhibits to answer the question. What are the names of missing classes XXX and YYY in exhibit below ? (choose one)
https://certify.torolab.ibm.com/figures/486fig5603137.gif 
a)XXX = DeliverySchedule, YYY = Supplier
b)XXX = Supplier, YYY = DeliverySchedule
c)XXX = PreferredSupplier, YYY = DeliverySchedule
d) XXX = DeliverySchedule, XXX = PreferredSupplier
YYY should be DeliverySchedule.
XXX should be either Supplier or PreferedSupplier. But, since it is generalization and not Interface and since Supplier is not abstract, I chose (b) to be the correct answer.
Please comment.