A．Because they get low grades.
B．Because they exhaust the reference sources in the library.
C．Because they can spend only limited time with the professors.
D．Because their teachers do not grade every assignment of them.

A．The roads are too crowded to drive.
B．It’s difficult to catch a bus or a train.
C．They are too young or too old to drive.
D．Train and bus services are becoming poorer.