【阅读理解】The structure of a family takes different forms around the world and even in the same society. The family's form changes as it adapts to changing social and economic influences. Until recently, the most common form in North America was the nuclear family, consisting of a married couple with their minor children. The nuclear family is an independent unit. It must be prepared to fend for itself. Individual family members strongly depend on one another. There is little help from outside the family in emergencies. Elderly relatives of a nuclear family are cared for only if it is possible for the family to do so. In North America, the elderly often do not live with the family; they live in retirement communities and nursing homes.
The information in the first paragraph is presented mainly through _____.