What entities, relationships, and conditions$($mandatory$/$optional$)$ are established by the following

A customer may generate many invoices.

An invoice is generated by only one customer.

b$.(3$pts$)$

A training session must be scheduled for more than $10$ employees and less than $30$ employees.

An employee can enroll in one or more training sessions.

c$.(3$pts$)$

A student may take up to six classes.

A class is limited to thirty$-$five students.

d$.(3$pts$)$

An agent may assist many customers.

A customer must be helped by only one agent.