The possessive pronoun "their" in the last paragraph is used in place of ______.

A．All interaction between teachers and students is accomplished "online" through an Internet or intranet connection.
B．Learning at home.
C．A system of learning on your own by writing letters.
D．A learning system for college and university students.

A．it is not as interesting as face-to-face learning
B．the learners only have the written text
C．some students don't know how to organize their time
D．some learners don't need face-to-face cues