There will be a steady trend toward vegetarianism. A given quantity of ground can provide plant food for man or it can provide plant food for animals which are later killed for meat. In converting the tissues of food into the tissues of the feeder, up to 90 per cent is used for reasons other than tissue maintenance and growth. This means that one hundred pounds of plant food will support ten pounds of human tissue―while one hundred pounds of plant food will support ten pounds of animal tissue, which will then support one pound of human tissue. In other words, land devoted to plant food will support ten times as many human beings as land devoted to animal food. It is this (far more than food preferences or religious directions ) that forces overcrowded populations into vegetarianism. And it will be the direction in which the United States of 2001 will be moving―not by presidential order, but through the force of a steady rise in meat prices as compared with other kinds of food. This, in turn, will come about because our herds will decrease as the food demand causes more and more meadow to be turned to farmland, and as land producing corn and other animal food is converted to providing food directly for man. Another point is that it is not only energy that is in short supply. A shortage of oil means a shortage of plastics; a shortage of electricity means a shortage of aluminium. We are also experiencing a shortage of paper and most other raw materials. This means that, for one thing, our generosity in wrapping, bagging and packaging will have to recede. There will have to be at least a partial return in supermarkets to the old days where goods were supplied in bulk and given out in bags to order. It may even become necessary to return bags, as we once returned bottles, or pay for new ones. A decline in per-capita energy use will make it necessary to resort to human muscle again, so that the delivery man will make a comeback (his price added to that of the food, of course).Since energy shortages will cause unemployment in many sectors of the economy, there will be idle hands to do the manual work that will become necessary. From an energy-saving standpoint, it would make far more sense to order by phone and have a single truck deliver food to many homes, than for a member of each home to drive an automobile, round-trip, to pick up a one-family food supply. To be sure, it will not all be retrogression. Even assuming that Earth is in a desperate battle of survival through a crisis of still rising population and dwindling energy reserves, there should still continue to be technological advances in those directions that don’’t depend on wasteful bulk use of energy. There will be continuing advances in the direction of "sophistication", in other words. The shortage of energy will result in
A. a steady decrease in the feeding of herds.
B. a continual drop in recycling used packages.
C. a forced return to an early stage of certain jobs.
D. a great reduction in the kinds of motor vehicles.
The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The success of the immune system in defending the body relies on a dynamic regulatory communications network consisting of millions and millions of cells. Organized into sets and subsets, these cells pass information back and forth like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produces an immune response that is prompt, appropriate, effective and self-limiting. At the heart of the immune system is the ability to distinguish between self and non-self. When immune defenders encounter cells or organisms carrying foreign or non-self molecules, the immune troops move quickly to eliminate the invaders. Virtually every body cell carries distinctive molecules that identify it as self. The body’’s immune defenses do not normally attack tissues that carry a self marker. Rather, immune cells and other body cells coexist peaceably in a state known as self-tolerance. When a normally functioning immune system attacks a non-self molecule, the system has the ability to remember the specifics of the foreign body. Upon subsequent encounters with the same species of molecules, the immune system reacts accordingly. With the possible exception of antibodies passed during lactation ( 哺乳期 ), this so-called immune system memory is not inherited. Despite the occurrence of a virus in your family, your immune system must learn from experience with the many millions of distinctive non-self molecules in the sea of microbes in which we live. Learning necessitates producing the appropriate molecules and cells to match up with and counteract each non-self invader. Any substance capable of stimulating an immune response is called an antigen. Tissues or cells from another individual (except an identical twin, whose cells carry identical self-markers ) act as antigens; because the immune system recognizes transplanted tissues as foreign, it rejects them. The body will even reject nourishing proteins unless they are first broken down by the digestive system into their primary, non-antigenic building blocks. An antigen announces its foreignness by means of intricate and characteristic shapes called epitopes( 抗原决定基), which stick out from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitopes on their surface, some may even carry several hundred. Some epitopes will be more effective than others at stimulating an immune response. Only in abnormal situations does the immune system wrongly identify self as non-self and execute a misdirected immune attack. The result can be so-called autoimmune disease. The painful side effects of these diseases are caused by a person’’s immune system actually attacking itself A tissue transplanted from father to daughter would be less acceptable than that transplanted between twins because
A. the ages of the twins’’ tissues are exactly alike.
B. the twins’’ tissues bear the same self-markers.
C. the father and daughter are different in sex.
D. the twins’’ molecules possess identical memory.
如图所示重力式挡土墙,墙背直立、光滑、填土面水平,墙高5m,墙体砌体重度γk=22kN/m3,作用在墙上的主动土压力Ea=76kN/m。 按《建筑地基基础设计规范》(GB5007-2002),挡土墙抗滑移稳定性满足()。
A. Ks=2.26≥1.35 满足
B. Ks=1.13≤1.5 不满足
C. Ks=0.955≤1.3 不满足
D. Ks=1.13≤1.5 不满足
某砌体承重结构,底层砖墙370mm厚,采用钢筋混凝土条形基础。地质条件如下:地面下3.5m厚黏土层,物理性质指标:γ=18.6kN/m,ωL-=39.4%,ωp=23.2%,ω=26.6%,ds=2.70,Es=6.9MPa,fak=200kPa。其下为淤泥质土层,物理性质指标为:ω=52%,Es=2.3MPa,fak=76kPa。上部结构荷载效应为:正常使用极限状态下的荷载效应的标准组合:Fk=210kN/m,Mk=40kN·m承载能力极限状态下的荷载效应的基本组合:Fk=235kN/m,Mk=55kN·m试设计该基础(基础底面埋深可取2.0m,基础混凝土强度等级C20;100厚混凝土垫层C10;混凝土轴心抗拉设计强度ft=1.1N/mm2;钢筋采用HPB235,钢筋强度设计值:fy=210N/mm2)。 若基础宽度b=1.8m,在最大基底压力一侧,基底边缘最大净反力Pjmax=256kPa,在墙边缘基底净反力Pj=125kPa。计算该基础底板承受的最大弯矩最接近()。
A. 54.3kN·m
B. 86.5kN·m
C 106.5kN·m
D. 148.3kN·m