Entity Relationship Model: Ternary Relationships

I am trying to understand why this statement in the book is wrong: "given a C entity, there is at most one related A entity and at most one related B entity". Is it that it doesn't apply to a specific kind of relationship?? So, if I have an example of a student who is in attendance to a course with a type of subject. The entities are student, attendance, course and s开发者_运维技巧ubject. Student makes attendance in a room. Also, a student can make attendance for a subject. Does this example apply to the statement?

Thanks for your time.

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The author probably means: in a single relationship instance (e.g., student-Bob/course-ABC/attendance123). So there is a single student, a single course and a single attendance record linked in that instance.

Not across all relationship instances in the class (where student Bob could attend many classes over time).