A、To attack the criminals when he finds them. B、To patrol in the cities. C、To sit down when he finds drugs. D、To scratch and dig

The dog has often been an unselfish friend to man. It is always grateful to its masters. It helps man in many ways. Certain breeds of dogs are used in criminal investigations. They are trained to sniff out drugs and bombs. They help police to catch criminals. The dogs that help in criminal investigations are trained at a school called the Military Dog Studies Branch of the US Air Force in Texas. The dogs to be trained are selected by an air force team. This team visits large cities across the country to buy the dogs. They may buy dogs from private citizens for up to $750 each.  
Some citizens freely give their dogs. The dogs selected must be healthy, brave and aggressive. They must be able to fight back if they are attacked. The dogs chosen are between the ages of one and three. They are given a medical examination when they arrive at the school. Their physical examination includes X-rays and heart tests. The trainee dogs undergo the first stage of training when they arrive. This is an 11-week course for patrol duty.  After this course, the best dogs are selected to go on to another 9-week course. They learn drug-sniffing or bomb-sniffing. After this course, the dogs are ready for their jobs in the cities or on air force bases. The training given to a drug-sniffing dog is different from that given to a bomb-sniffing dog. A drug-sniffing dog is trained to scratch and dig for the drugs when he sniffs them. A bomb-sniffing dog sits down when he finds a bomb. That is the alert for hidden explosives.
Questions 16 to 18 are based on the passage you have just heard.
16. How are the dogs obtained for training?
17. What is the first stage of training the dogs?
18. What is the content of training given to a drug-sniffing dog?