题目内容：

Women are friendly．But men are more competitive．Why? Researchers have found it's all down to the hormone oxytocin （荷尔蒙催生素）．Although known as the love hormone, it affects the sexes differently．

"Women tend to be social in their behavior．They often share with others．But men lend to be competitive．They are trying to improve their social status," said Professor Ryan．

Generally, people believe that the hormone oxytocin is let out in our body in various social situations and our body creates a large amount of it during positive social interactions （互动） such as falling in love or giving birth．

But in a previous experiment Professor Ryan found that the hormone is also let out in our body during negative social interactions such as envy．

Further researches showed that in men the hormone oxytocin improves the ability to recognize competitive relationships, but in women it raises the ability to recognize friendship．

Professor Ryan's recent experiment used 62 men and women aged 20 to 37．Half of the participants（参与者）received oxytocin．The other half received placebo （安慰剂）．

After a week, the two groups switched with participants．They went through the same procedure with the other material．

Following each treatment, they were shown some video pictures with different social interactions．Then they were asked to analyze the relationships by answering some questions．The questions were about telling friendship from competition．And their answers should be based on gestures, body language and facial expressions．

The results indicated that, after treatment with oxytocin, men's ability to correctly recognize competitive relationships improved, but in women it was the ability to correctly recognize friendship that got better．

Professor Ryan thus concluded: "Our experiment proves that the hormone oxytocin can raise people's abilities to better distinguish different social interactions．And the behavior differences between men and women are caused by biological factors （因素） that are mainly hormonal．"

1.What causes men and women to behave differently according to the text?

A. Placebo．    B. Oxytocin．

C. The gesture．    D. The social status．

2.What can we learn from Professor Ryan's previous experiment?

A. Oxytocin affects our behavior in a different way．

B. Our body lets out oxytocin when we are deep in love．

C. Our body produces oxytocin when we feel unhappy about others' success．

D. Oxytocin improves our abilities to understand people's behavior differences．

3.Why did Professor Ryan conduct the recent experiment?

A. To test the effect of oxytocin on the ability to recognize social interactions．

B. To know the differences between friendship and competition．

C. To know people's different abilities to answer questions．

D. To test people's understanding of body language．

4.The author develops the text by______．

A. explaining people's behaviors

B. describing his own experiences

C. distinguishing sexual differences

D. discussing research experiments