Super Sense of Small
①Dogs have a surprising sense of smell. It allows them to perform excellently at search-and-rescue operations. It also enables them to detect bombs. They can even sniff（嗅）out chemical changes resulting from certain diseases. Therefore, dogs are trained to save people's lives.
②Dogs begin training when they are two months old. Their owners drop them off at training centers from 9 am to 5 p. m. every weekday. During the time, trainers play hide-and-seek with the dogs to train their sense of smell. They teach the dogs to climb ladders and run past obstacles（障碍）They also expose the dogs to loud noises.
③Diseases have specific smells. Although the smells are usually too subtle for human noses to detect, dogs can tell something is wrong and warn people about the problem by barking. Many people have reported that their dogs warned them of an illness.
④Scientists are training dogs to detect disease. A dog is asked to sniff a wheel. Blood samples from different people are attached to the outside of the wheel. Blood from a cancer patient has a slightly different smell than blood from a healthy person. The dog must figure out which blood sample to pick.
⑤Scientists are working to design an electronic nose that can sniff out cancer. To do that, they need to figure out what molecules（分子）are sent out from the cancer. Then they can create a mixture with similar smell to that of the cancer. Once scientists know which molecules match the cancer's smell, e-nose could be designed to detect cancer.
Dogs are trained to detect diseases_____.
A、to train their sense of smell
B、to perform search and rescue
C、which are caused by diseases
D、who suffer from certain cancers
E、which molecules go with cancer
F、by recognizing certain blood samples
【正确答案】：F
【题目解析】：第四段表明，狗被训练从多个血液样本中嗅出得了癌症的人的血样，这是狗被训练检测疾病的训练方式。因此选择F选项。
