单项选择题

Directions: There are five reading passages in this part. Each passage is followed by four questions. For each question there are four suggested answers marked A, B, C and D. Choose the best answer and blacken the corresponding letter on the Answer Sheet.
Passage One

Many animals have an excellent sense of smell, which they use in hunting. On the other hand, their eyesight may be poor. Dogs, for example, have poor eyesight and no color vision. They see only shades of gray. But the dog’s sense of smell is extraordinary, The kind of dog known as Alsatian has 220 million olfactory (嗅觉) cells. Man has five million. Scientists believe that the Alsatian is one million times better than man in finding out odors. The human sense of smell, however, is really quite good. The average human being can distinguish more than 10,000 different odors.

The best title of the passage is"（）".

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