The word "Their" in Line 5, Para. 2 refers to______.

A．to define corporations and Firms
B．to show the relation between firms and colleges
C．to inform the job-seeker of the employment requirements
D．to give a description of corporations for college students

A．They only employ graduate students in colleges.
B．They can offer many job opportunities.
C．They have many sub-companies throughout the country.
D．Their requirements are very high.