important question on Entity Relationship Model(ER Model)

5)A university registrar’s office maintains data about the following entities: (a) courses, including number, title, credits, syllabus, and prerequisites; (b) course offerings, including course number, year, semester, section number, instructor(s), timings, and classroom; (c) students, including student-id, name, and program; and (d) instructors, including identification number, name, department, and title. Further, the enrollment of students in courses and grades awarded to students in each course they are enrolled for must be appropriately modeled.
Construct an E-R diagram for the registrar’s office. Document all assumptions that you make about the mapping constraints.
Answer: 

 In the answer given here, the main entity sets are student, course, course-offering, and instructor. The entity set course-offering is a weak entity set dependent on course. The assumptions made are :

• a class meets only at one particular place and time. This E - R diagram cannot model a class meeting at different places at different times.
• There is no guarantee that the database does not have two classes meeting
  at the same place and time.

6)Construct an E-R diagram that models exams as entities, and uses a ternary relationship, for the above database.
Answer : 

7)  Construct an alternative E-R diagram that uses only a binary relationship between students and course-offerings. Make sure that only one relationship exists between a particular student and course-offering pair, yet you can represent the marks that a student gets in different exams of a course offering.
answer :

8) Explain the difference between a weak and a strong entity set.
Answer: A strong entity set has a primary key. All tuples in the set are distinguishable by that key. A weak entity set has no primary key unless attributes of the strong entity set on which it depends are included. Tuples in a weak entity set are partitioned according to their relationship with tuples in a strong entity.