Read the following passage and choose the best answer (A, B, C, D):
There are many types of family systems around the world. In North America and northern Europe, the nuclear family (with two generations - a father, a mother and one or more children) is often seen as the most typical. In contrast, in most other parts of the world, extended families, which include other family members such as grandparents, aunts, uncles and cousins, are seen as the norm. The common view is that the nuclear family has become the norm in many Western societies as a result of industrialization and urbanization. This trend began in the late eighteenth and nineteenth centuries, when people were forced to move to cities to find work in the factories that sprang up during the Industrial Revolution. In the twentieth century, greater industrialization resulted in even more people leaving their large extended families. Urbanization also meant that people lived in much smaller houses, which were not big enough for an extended family. The trend towards nuclear families meant that many of the duties and responsibilities of a family, such as providing food and shelter, cleaning the home, preparing the food, caring for children and their education, and caring for the sick and elderly are no longer shared among the members of the extended family. The parents (or parent) now have to do this, with some help from the state. However, this is the price that people pay for the higher standard of living that may come from living in a city. We may think we know what we mean by a ‘nuclear’ family and an extended family, but reality is more complicated than most people believe. Most nuclear families are part of extended families: children have grandparents and, in many cases, aunts, uncles and cousins as well. Part of what makes them ‘nuclear’ is that they live in their own separate household, but it is not the whole story. In Greece or Italy, for example, a nuclear family may live in its own flat, but the extended family may live in the same apartment block or in the same street and family members see each other and even eat together every day. There is at least one more factor to consider. Family members may be separated from each other by geographical distance, but they may have close emotional ties. Even in North America and northern Europe, grandparents usually have close bonds with their grandchildren, and families often travel long distances so that they can see each other. Grandparents often help their adult children, for example, by cooking and looking after their children in emergencies. In the same way, when their parents become too old to live on their own, adult children may take them into their own homes. As a result, they turn their nuclear family into an extended family. The structure of families changes over time. The effects of urbanization and industrialization are enormous, but they are not the only reasons for the changes. People marry, have children, become widowed, divorce and die. Children grow up and adults grow old. Nuclear families become extended families and extended families become nuclear families. Family ties stay strong or become weak. One thing is certain: in a changing world, the family will continue to change, but ultimately, it is likely to continue to be the basic unit of society
6. The word "their" in the passage refers to ____
