18. In a relational database table, a foreign key column can contain: (a) NULLS or blanks (b) duplicate values (c) both a and b above (d) neither a nor b above 19. A small college database contains INSTRUCTOR and CLASS entities/tables. Instructor-ID; PK of CLASS table is Class-Code. To create a relationship between these two tables, what would PK of INSTRUCTOR table is you do? (a) include Instructor-ID column in CLASS table as a foreign key column (b) include Class-Code in INSTRUCTOR table as a foreign key column (c) place a third table between INSTRUCTOR and CLASS tables 20. In ER diagrams, the term "many" means: (a) any number zero or more (c) any number including zero, less than zero, and more than zero (b) any number one or more