precarious（）

seeds and fruits. Not long after the last Ice Age, around 7,000 B.C. (during the Neolithic period), some hunters and gatherers began to rely chiefly on agriculture for their sustenance. Others continued the old pastoral and nomadic ways. Indeed, agriculture itself evolved over the courseof (5) time, and Neolithic peoples had long known how to grow crops. The real transformation of human life occurred when huge numbers of people began to rely primarily and permanently on the grain they grew and the animals they domesticated. Agriculture made possible a more stable and secure life. With it Neolithic peoples flourished, fashioning an energetic, creative era. They were responsible for many fundamental inventions and (10) innovations that the modern world takes for granted. First, obviously, is systematic agriculture-- that is, the reliance of Neolithic peoples on agriculture as their primary, not/nerely subsidiary, source of food. Thus they developed the primary economic activity of the entire ancient world and the basis of all modern life. With the settled routine of Neolithic farmers came the evolution of towns and (15) eventually cities. Neolithic farmers usually raised more food than they could consume, and their surpluses permitted larger, healthier populations. Population growth in turn created an even greater reliance on settled farming, as only systematic agriculture could sustain the increased numbers of people. Since surpluses o food could also be bartered for other commodities, the Neolithic era witnessed ihe beginnings of large-scale exchange of goods. In time the increasing (20) complexity of Neolithic societies led to the development of writing, prompted by the need to keep records and later by the urge to chronicle experiences, learning, and beliefs. The transition to settled life also had a profound impact on the family. The shared needs and pressures that encourage extended-family ties are less prominent in settled than in nomadic societies. Bonds to the extended family weakened. In towns and cities, the nuclear family was (25) more dependent on its immediate neighbors than on kinfolk. What does the passage mainly discuss？ A．Why many human societies are dependent on agriculture B．the changes agriculture brought to human life C．How Neolithic peoples discovered agriculture D．Why the first agricultural societies failed