单选题 The dogs that help in criminal investigations are trained at a school called the Military Dog Studies branch of the US Air Force in Lackland, Texas. The dogs to be trained are selected by an air force team. This team visits large cities across the country to buy 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 in Lackland. This is an 11 - week course for patrol duty. After this course, the best dogs are selected to go on 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 off air force bases.
Dogs are ready for their jobs of drug - sniffing or bomb - sniffing

• A. after an 11 - week course.
• B. after a 9 - week course.
• C. after they are given medical examinations
• D. after they have grown large enoug