After having dinner in a restaurant, the visitor plans to take the last train ______

A．By passing steam over dry ice.
B．By turning ordinary ice into steam.
C．By melting dry ice.
D．By mixing dry ice with ordinary ice.

A．Because they all work very hard.
B．Because their teachers are all native speakers of English.
C．Because they learn not only in but also out of class.
D．Because they are all advanced students.