When required in tonnage quantities, oxygen is prepared by the’ fractional distillation of liquid air. The process takes advantage of the fact that when a ______ gas is allowed to expand, it cools. (compress)

A．it ignores the fact that drugs vary greatly in danger
B．it gives a false account of the risks of drug use
C．some drugs are good for health
D．it does not appear in underground papers

A．but most of them wish to stay away from it
B．and many of them hope to be interviewed some day
C．and many of them would like to acquire a true understanding of it
D．but most of them may not have been interviewed in person