6．
Have you ever thought of why dogs are human's best friend？What's the bond（紧密关系）between Human and a dog？A new study by Japanese scientist Miho Nagasawa seem to have found the answer，and it has to do with something named the cuddle（拥抱）chemical．
The cuddle chemical has another，more scientific name：oxytocin（催产素）．Oxytocin is something in the blood that encourages bonding．Levels of oxytocin increase，for example，when a mother feeds her newborn baby．According to Nagasawa's study，levels of oxytocin can also go up when we look deeply into the eyes of a dog．
Humans and dogs have been working together for nearly 30，000years．In order to better understand how the cuddle chemical worked between dogs and humans，Nagasawa and his team did an experiment．They tested levels of oxytocin in dogs and humans，and then put them in a room to interact（交流）with each other．While in the room，the humans pet the dogs，spoke to the dogs，and looked into the dogs'eyes．Then the scientists tested their levels of oxytocin again．
The researchers found that levels of oxytocin were higher in both humans and dogs after they interacted．However，levels were the highest in the humans and dogs that simply looked into each other's eyes，without much petting or talking．The longer the humans looked into the eyes of the dogs，the more cuddle chemical let out．Nagasawa did the same experiment with humans and wolves（狼），but it did not give the same results．
52．The underlined word"it"in Paragraph 1refer toB．
A．something
B．the answer
C．a new study
D．the cuddle chemical
53．Nagasawa did the experiment of humans and dogs to knowA．
A．how oxytocin worked
B．how oxytocin was divided
C．how they helped each other
D．how they interacted with each other
54．Oxytocin levels reached the highest in the humans and dogsA．
A．when they looked into each other's eyes
B．when they talked with each other
C．if the dogs cuddled the humans
D．if the humans pet the dogs
55．What can we infer（推断）from the last sentence of the text？D
A．More experiments are expected to help wolves produce oxytocin．
B．The cuddle chemical in human blood protects us against wolves．
C．It makes sense that humans and wolves are naturally best friends．
D．It's generally impossible to have the bond between humans and wolves．
