若x，y满足约束条件，则z=x-2y的最小值为（）。

The effect of the baby boom on the schools helped to make possible a shift in thinking about the role of public education in the 1920’s. In the 1920’s, but especially (1) the Depression of the 1930’s, the United States experienced a (2) birth rate. Then with the prosperity (3) on by the Second World War and the economic boom that followed it, young people married and (4) households earlier and began to (5) larger families than had their (6) during the Depression. Birth rates rose to 102 per thousand in 1946, 106.2 in 1950, and 118 in 1955. (7) economics was probably the most important (8) , it is not the only explanation for the baby boom. The increased value placed (9) the idea of the family also helps to (10) this rise in birth rates. The baby boomers began streaming (11) the first grade by the mid-1940’s and became a (12) by 1950. The public school system suddenly found itself (13) The wartime economy meant that few new schools were buih between 1940 and 1945. (14) , large numbers of teachers left their profession during that period for better-paying jobs elsewhere.

(15) , in the 1950’s, the baby boom hit an antiquated and inadequate school system. Consequently, the custodial rhetoric of the 1930’s no longer made (16) ; keeping youths ages sixteen and older out of the labor market by keeping them in school could no longer be a high (17) for an institution unable to find space and staff to teach younger children. With the baby boom, the focus of educators (18) turned toward the lower grades and back to basic academic skills and (19) . The system no longer had much (20) in offering nontraditional, new, and extra services to older youths.

14().

A. Moreover

B. However

C. Otherwise

D. Thus

某奶牛养殖场月初有30 只母牛和10 只小牛，每天共需饲料650 kg ，10 天后又买进50 只母牛和20 只小牛，这时每天共需饲料1750 kg ，每千克饲料为2 元．

  (1) 求平均每天每只母牛和每只小牛各需饲料多少千克．

  (2) 该养殖场这个月（按30 天计算）共需饲料费用多少元？

  (3) 若饲养每只牛每天人工费5 元，每只母牛每天产奶20 kg ，每千克牛奶售价3 元，小牛都不产奶，求从月初起至少要多少天该养殖场获得利润不少于10 500 元（不计算买牛的费用）．

在同一条等响曲线上，如噪声听起来与频率为1000Hz的声压级60dB的基准纯音一样响，则该噪声的响度级为(　　)方。　　

A．0　　
B．120　　
C．20　　
D．60

FDDI与Token Ring都采用 (24) 传递协议，在FDDI的令牌帧中有 (25) ，其主要作用是 (26) 。FDDI在 (27) 产生新令牌帧，允许在环上同时存在 (28) 。

28（）

A．1个令牌帧和1个数据帧

B．2个令牌帧

C．1个令牌帧

D．2个数据帧