If you’ve ever thought that dogs seem to know when people are stressed, you were right. Scientists did an experiment which showed that dogs can tell the difference between the smell of a person when they’re relaxed and when they’re stressed.

Researchers worked with four dogs. They taught the dogs to use a special smell-testing device(装置) with three openings. The scientists used treats and rewards to train the dogs to find the correct opening. Over time, the dogs were given harder and harder smelling jobs. The final goal was to see if they could smell stress in the breath and sweat of a person.

All day long, our bodies go through changes and produce different chemicals(化学物). These chemicals come out in our breath and sweat, slightly changing the way we smell. But it wasn’t known if stress could cause changes that could be discovered. 

So the researchers collected breath and sweat samples(样本) from 36 different people. They got samples when the people were calm, and also when they were stressed. Then the researchers collected two more sweat and breath samples. The scientists used one of the stressed samples, along with two clean pieces of cloth, to train the dogs to find the stressed smell. 

Then came the challenge—the dogs had three choices: a sample from a calm person, a stressed sample from that same person, and a clean piece of cloth. In all, the dogs did 720 tests. They correctly identified the stressed sample about 94% of the time.

The results make it clear that stress does have a smell that dogs can find. Scientists say the news could be especially useful in training service dogs, since their job is to help relieve that stress.

1.What’s the purpose of the experiment?

A To know when people are stressed.

B To learn if dogs can smell stress.

C To teach dogs to find the correct opening.

D To check the breath and sweat of people.

解析：选B。B细节理解题。根据第二段的“The final goal was to see if they could smell stress in the breath and sweat of a person.”可知，该实验的目的是看狗能否从人类的呼吸和汗水中嗅出压力。故选B。

2.Why did the scientists use samples of breath and sweat to test the dogs?

A Because our bodies produce different sweat every day.

B Because stress could be changed by sweat and breath.

C Because our bodies become stronger day by day.

D Because they make our bodies smell different.

解析：选D。D细节理解题。根据第三段的第一、二句可知，人体一整天都在产生不同的化学物，这些化学物通过呼吸和汗水释放出来，略微改变了我们的气味。所以科学家用呼吸和汗水的样本来测试狗狗。故选D。

3.What does Paragraph 4 mainly talk about?

A The importance of the experiment.

B The influence of the experiment.

C The process of the experiment.

D The result of the experiment.

解析：选C。C段落大意题。通读第四段内容可知，该段主要描述了科学家是如何进行该实验的，即实验过程。故选C。

4.The underlined word “identified” can be replaced by ________.

A provided

B exchanged

C recognized

D prevented

解析：选C。C词义猜测题。根据最后一段“The results make it clear that stress does have a smell that dogs can find.”可知，该研究表明狗狗能嗅出压力，由此可推知在研究中狗在94%的时间里都正确地“识别”了压力样本。故选C。

5.The passage is probably from ________.

A a travel website

B a science magazine

C a history book

D science fiction

解析：选B。B推理判断题。根据全文内容可知，本文是关于狗能否嗅出人类压力的科学研究的文章，属于“科学”版块，最有可能在科学杂志上读到。故选B。