American's city dwellers are a mobile people. The decennial censuses provide documentation

in their redundant accounts of rapid changes and growth in most of our great cities. But statistical evidence is hardly needed. The changes in our cities have occurred so rapidly that the perception of mobility is an integral formed in the intervals between visits. The neighbour-hoods of our childhood present alien appearances and the landmarks that anchored our memories have disappeared.

How do these dramatic changes in residential areas come about？ In part, industry and commerce in their expansion encroach upon land used for residence. But, in larger part, the changes are mass movements of families--the end results of countless thousands of residence shifts made by urban Americans every year. Compound in the mass, the residence shifts of urban households produce most of the change and flux of urban population structures.

Some of the mobility is an expression of the growth of our population. Every new family started ordinarily means another household formed. The mobility that occurs is much greater than can be accounted for only by an addition of new households to our population. The high level of mobility implies that established households are involved in a large-scale game of "musical chairs" in which housing is exchanged from time to time.

Residential shifts often accompany the dissolution of households, although not as consistently as in the case of the formations of new households. A divorce or separation forces at least one to move, and often both husband and wife Shift residence. Morality sometimes precipitates a move on the part of the remaining members of the household. But, neither divorce nor morality, when added to new household formation, can account for more than a very small part of the American mobility rate.

Another part of the high residential mobility rate might be traced to changes occurring in the labour force. American workers change jobs frequently and some of the residential mobility might be viewed as consequence of job shifts that do not involve long-distance movements. About three fourths of such shifts do not cross country boundaries and many of them take place within smaller areas. Neither can job shifts account for the overall picture of mobility, much of which is a kind of "milling out" within small areas of the city.

The phrase "encroach upon" in Paragraph 2 most probably means ______.

A．buy and sell

B．measure and survey

C．invade and occupy

D．rent and share