题目内容：

By trying to tickle(挠痒痒) rats and recording how their nerve cells respond, Shimpei Ishiyama and his adviser are discovering a mystery that has puzzled thinkers since Aristotle expected that humans, given their thin skin and unique ability to laugh, were the only ticklish animals.

It turns out that Aristotle was wrong. In their study published on Thursday, Ishiyama and his adviser Michael Brecht found that rats squeaked and jumped with pleasure when tickled on their backs and bellies. These signs of joy changed according to their moods. And for the first time, they discovered a special group of nerve cells. These nerve cells made this feeling so powerful that it causes an individual being tickled to lose control.

To make sure that he had indeed found a place in the brain where tickling was processed, Ishiyama then stimulated(刺激) that area with electrical currents. The rats began to jump like rabbits and sing like birds.

“It’s truly ground-breaking,” said Jeffrey Burgdorf, a neuroscientist at Northwestern University who reviewed the paper. “It takes the study of emotion to a new level.”

Burgdorf has played a central role in our understanding of animal tickling. He was part of a team that first noticed, in the late 1990s, that rats made special noises when they were experiencing social pleasure. Others had already noted that rats repeatedly made short and high sounds during meals. But the lab where Burgdorf worked noticed that they emitted similar sounds while playing. And so one day, the senior scientist in the lab said, “Let’s go and tickle some rats.” They quickly found that those cries of pleasure doubled.

“The authors have been very adventurous,” said Daniel O’Connor, a neuroscientist at Johns Hopkins University who studies touch. To him, that finding was very surprising.

“Why does the world literally feel different when you are stressed out?” he said. “This is the first step towards answering that question. It gives us a way to approach it with experimental rigor(严谨).”

1.What contributed to humans’ being ticklish according to Aristotle?

A. Their special skin.    B. Their social pleasure.

C. Their nervous system.    D. Their willingness to touch.

2.Why did the researchers make use of electrical currents?

A. To discover the special group of nerve cells.

B. To experiment on different animals.

C. To follow the process of tickling.

D. To prove their finding.

3.Which of the following statements will Jeffrey Burgdorf agree with?

A. The research process is full of risks.

B. The finding of the study is surprising and unbelievable.

C. The new discovery is beneficial for the study of emotion.

D. The finding of the study actually contradicts modern science.

4.The underlined word “squeak” in Paragraph 2 may be explained by ______.

A. give a smile    B. make a noise

C. burst into tears    D. watch with staring eyes

5.What is the best title for the text?

A. The Life of Rats    B. How Rats Laugh

C. A Wonderful Scientist    D. A New Discovery about Rat