If you've ever thought that dogs seem to know when people are stressed，you were right.Scientists carried out a study，which showed that dogs can tell the （1）

difference

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（裝置）（2）

with

with

three openings.The scientists used treats to train1 the dogs to find the correct opening.Over time，the dogs were given harder and harder smelling jobs.The final goal was to see（3）

whether

whether

they could smell stress in the breath and sweat（汗水） of a person.
   All day long，our bodies go through changes and produce different chemicals（化學物）.These chemicals come out in our breath and sweat，changing the way that we（4）

smell

smell

.But it wasn't known if stress could cause changes that could be discovered.
   So the researchers collected breath and sweat samples（樣本） from 36 different people.They got samples when the people felt （5）

relaxed

relaxed

，and also when they were under great stress.Then the researchers （6）

used

used

the samples and a clean piece of cloth to train the dogs to find the stressed smell.
   Then the dogs had to（7）

deal

deal

with one challenge.They had three choices：a sample from a calm person，a stressed sample from that same person，and a clean piece of cloth.In total，the dogs did 720 tests.They correctly find the stressed sample about 94% of the time.
   The results make（8）

it

it

clear that stress does have a smell that dogs can discover.Scientists say the findings could be especially useful in training service dogs，since their job is to help lower that stress.