Read the passage, and choose the correct answer A, B, C or D for each question.

The American family unit is in the process of change. In the first half of the 20th century, there were mainly two types of families: the extended and the nuclear. An extended family includes mother, father, children and some other relatives, living in the same house. A nuclear family is composed of just parents and children living under the same roof.

As the American economy had progressed from agricultural to industrial one, people were forced to move to different parts of the country to get good jobs. These jobs were mainly in the large cities. Now, in fact, three-quarters of Americans live in urban areas which occupy 2.5% of the national total land mass. Of the 118 million in the labour force, only 3 million still work on the farm.

Since moving for better jobs has often divided the extended family, the nuclear family became more popular. At present, 55% of the families in the US are nuclear families. But besides the two types of traditional family groupings, the family is now being expanded to include a variety of other living arrangements because of divorce. There is an increase in single-parent families, in which a father or mother lives with one or more children. Divorce has also led to blended families, which occur when previously married men and women marry again and combine the children from former marriage into a new family. There are also some couples who do not want to have children to form two-person childless families.

A nuclear family is one that ______________.