Customs officers at a London airport yesterday found $500000 worth of drugs which were being smuggled into Britain in boxes marked "Urgent Medical Supplies. " The (51) might have suspected for some time that the drugs were being brought into the country in this way. The (52) is believed to be the work of a well-organized international group. Four men were arrested at the airport and held for questioning, (53) it is unlikely that they are organizers. In fact, they declared that they were (54) of what the boxes contained and had acted in good faith in bringing (55) into Britain. This is the third time in six months that attempts have been made to smuggle (56) goods through Customs by declaring them to be medical supplies. They are frequently (57) in special containers and a warning is given that they may be (58) if they are not handled with care. They are determined to put a (59) to this practice, said one of the Customs officers today. (60) is no way these people are going to get away with this any longer. We have the full cooperation of the International Police who are as anxious as we are to track down the main source of supply.
52（）。
A.operation
B.behavior
C.movement
D.development