A．Catching runaway criminals．
B．Scratching the hidden bombs．
C．Patrolling the dangerous town．
D．Drug-sniffing or bomb-sniffing．

A．Because without air the plane can't fly．
B．Because the plane needs air for its refueling．
C．Because the passengers' lives depend on air．
D．Because the passengers have paid for it．

A．The plane would not go forward．
B．The plane would explode．
C．The plane would fall down．
D．The plane would fly slowly．