Which of the following statements about group insurance underwriting principles is $($are$)$ true?

A$.$ If a plan is contributory, $100$ percent of the eligible employees must be covered.

B$.$ Ideally, there should be a flow of younger people into the group and older people out of the group.

C$.$ Employees should be required to remit premiums directly to the insurance company.

D$.$ The average age of the group should ideally increase over time.

E$.$ A group should be formed for the specific purpose of obtaining insurance.

F$.$ A flow of people through the group is optional.