5. (11.) What primary key characteristics are considered desirable? Explain why each characteristic is considered desirable - For full credit I expect 6 characteristics and a short explanation as to why each is desirable. 6. (12.) Under what circumstances are composite primary keys appropriate? Note there are only two circumstances. 7. (14.) When implementing a 1:1 relationship, where should you place the foreign key if one side is mandatory and one side is optional? Should the foreign key be mandatory or optional? 8. (15.) What are time-variant data, and how would you deal with such data from a database design point of view? 9. (16.) What is the most common design trap, and how does it occur