2017年12月英语四级（第1套 · 含听力）选词填空

Questions __26__ to __35__ are based on the following passage.
A rat or pigeon might not be the obvious choice to tend to someone who is sick, but these creatures have some __26__ skills that could help the treatment of human diseases.
Pigeons are often seen as dirty birds and an urban __27__ , but they are just the latest in a long line of animals that have been found to have abilities to help humans. Despite having a brain no bigger than the __28__ of your index finger, pigeons have a very impressive __29__ memory. Recently it was shown that they could be trained to be as accurate as humans at detecting breast cancer in images.
Rats are often __30__ with spreading disease rather than __31__ it, but this long-tailed animal is highly __32__ . Inside a rat's nose are up to 1,000 different types of olfactory receptors (嗅觉感受器), whereas humans only have 100 to 200 types. This gives rats the ability to detect __33__ smells. As a result, some rats are being put to work to detect TB (肺结核). When the rats detect the smell, they stop and rub their legs to __34__ a sample is infected.
Traditionally, a hundred samples would take lab technicians more than two days to __35__ , but for a rat it takes less than 20 minutes. This rat detection method doesn't rely on specialist equipment. It is also more accurate — the rats are able to find more TB infections and, therefore, save more lives.