Which best serves as the title for the passage?

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 39 to 43.
A team of social psychologists from California has spent six years studying the reactions of people in cities around the world to different situations. The results show that cities where people have less money generally have friendlier populations. Rio de Janeiro in Brazil, which is often known for its crime, comes out top, and the capital of Malawi, Lilongwe, comes third.
But what makes one city friendlier than another? The psychologists from California State University say it has got more to do with the environment than culture or nationality.
They carried out a study into the way locals treated strangers in 23 cities around the world. The team conducted their research through a series of tests, where they dropped pens or pretended, they were blind and needed help crossing the street.
The study concludes that people are more helpful in cities with a more relaxed way of life such as Rio. While they were there, researchers received help in 93 percent of cases, and the percentage in Lilongwe was only a little lower. However, richer cities such as Amsterdam and New York are considered the least friendly. Inhabitants of Amsterdam helped the researchers in 53 percent of cases and in New York just 44 percent. The psychologists found that, in these cities, people tend to be short of time, so they hurry and often ignore strangers.
Which best serves as the title for the passage?
A. What makes a city population less friendly
B. The world's friendliest city
C. The environment and culture of a city
D. The research of psychologists about different cities