Which of the Following Is an Example of External Sources

Which of the following is an example of external sources of recruitment?

A) A veterinarian selects one of her interns as her assistant.
B) Jolene recommends her colleague to fill an open position as a sales representative at the company where Jolene works as a technician.
C) An assistant librarian is shortlisted for the post of head librarian in the same library.
D) A salon shifts from Boston to New York and employs a new cashier through Internet recruitment.