Given the question: Read the article and select the best answer. Article: Let's Learn More about Each Other Are the lives of city kids the same as those in villages? In many ways, they are very different. But what are the differences? Hu Peng of Wuhan and four friends decided to find them out. Earlier this month, they lived for a week in Caidian, a village near Wuhan. Hu and his friends went door to door in Caidian and asked the kids there lots of questions. They wanted to learn more about village kids' everyday lives, so they asked these questions like --Do your parents teach you how to do housework? How much money do you usually spend on dinner? What would you do if a thief   came into your house? They also asked 150 city kids the same questions.What's the Difference? Last week the team gave a report to their class. There are lots of differences between children's lives in cities and those in villages. The biggest difference is independence. Hu's team said that 60 percent of city kids couldn't do much housework, but 90 percent of village kids could care for themselves. City kids told Hu they cared about school a lot, and they had no time for washing clothes or making their beds. Village kids said they helped their parents with housework a lot. They cooked, did some cleaning and fed animals. Hu's team also found that village kids had less pocket money  . Many never used computers. They played in rivers or on mountains. Some didn't really like to make new friends. Hu and his friends said their trip gave them more self-confidence   because they were doing something by themselves. But _ also made them a little worried because they saw they still had a lot to learn. "When we grow up, our parents can't take care of us," said Hu. "We have to learn to take care of ourselves." Shen Guanquan, one of Hu's friends said, "When children learn to care for themselves, they learn to do lots of things." Question: From the article, the biggest difference is   _  between city kids and village kids. Options: A: friends B: Time C: self-confidence D: independence
The answer is:
D