1 . Despite common concerns that the social is competitive, cooperation (合作) among strangers has gradually increased in the U.S. since the 1950s, according to the research published by the American Psychological Association.

“We were surprised by our findings that Americans became more cooperative over the last six decades because many people believe U.S. society is becoming less socially connected and less trusting,” said lead researcher Yu Kou, PhD, a professor of social psychology at Beijing Normal University.

The researchers analyzed 511 studies conducted in the United States between 1956 and 2017 with a total of more than 63,000 participants(参与者). Those studies included lab experiments measuring cooperation among strangers. The study found a small, gradual increase in cooperation across the 61-year period, which the researchers said may be linked to great changes in U.S. society. The increase in cooperation was related with increases in social wealth, income inequality and the number of people living alone.

Increased cooperation has been linked with market competitiveness and economic growth in former research. As more people live in cities and on their own, they may be forced to cooperate with strangers, said study co-author Paul Van Lange, PhD. He said, “U.S. society may have become more self-centered, but people have not.”

The researchers note that former studies have found that levels of cooperation do not differ by sex or race in the U.S. However, the studies were conducted in lab settings primarily with only college students as participants, so the findings may not be representative (代表性的) of real-life situations or of U.S. society as a whole.

1. How did the researchers draw the conclusion?

A．By doing street surveys.

B．By conducting lab experiments.

C．By analyzing collected examples.

D．By interviewing different strangers.

2. What can we learn about the study from paragraph 3?

A．The finding was predicted.

B．The data before 1956 could not be found.

C．The cause for the increase in cooperation was not clear at all.

D．Increase in cooperation was connected with many fields of society.

3. Where is text probably from?

A．A magazine.

B．A newspaper.

C．A diary.

D．A website.

4. What might the researchers do next?

A．Conducting more experiments in the real-life situations.

B．Finding the period when cooperation among strangers increased.

C．Making sure levels of cooperation may be different by sex or race in the U.S.

D．Discovering the connection between increase in cooperation and changes in U.S. society.