共用题干
New Understanding of Natural Silk's Mysteries
Natural silk,as we all know,has a strength that man-made materials have long struggled to match.In a
discovery that sounds more like an ancient Chinese proverb than a materials science breakthrough,MIT re-
searchers have discovered that silk gets its strength from its weakness.Or,more specifically,its many weak-
nesses. Silk gets its extraordinary durability and ductility（柔韧性）from an unusual arrangement of hydrogen
bonds that are intrinsically very weak but that work together to create a strong,flexible structure.
Most materials一especially the ones we engineer for strength一get their toughness from brittleness.As
such,natural silks like those produced by spiders have long fascinated both biologists and engineers because of
their light weight,ductility and high strength(pound for pound,silk is stronger than steel and far less brittle).
But on its face,it doesn't seem that silks should be as strong as they are;molecularly,they are held together by
hydrogen bonds , which are far weaker than the covalent（共价的）bonds found in other molecules.
To get a better understanding of how silk manages to produce such strength through such weak bonds,
the MIT team created a set of computer models that allowed them to observe the way silk behaves at the
atomic level. They found that the arrangement of the tiny silk nanocrystals（纳米晶体）is such that the hydro-
gen bonds are able to work cooperatively,reinforcing one another against external forces and failing slowly
when they do fail,so as not to allow a sudden fracture to spread across a silk structure.
The result is natural silks that can stretch and bend while retaining a high degree of strength.But while
that's all well and good for spiders,bees and the like,this understanding of silk geometry could lead to new
materials that are stronger and more ductile than those we can currently manufacture.Our best and strongest
materials are generally expensive and difficult to produce(requiring high temperature treatments or
energy-intensive processes).
By looking to silk as a model,researchers could potentially devise new manufacturing methods that rely
on inexpensive materials and weak bonds to create less rigid,more forgiving materials that are nonetheless
stronger than anything currently on offer. And if you thought you were going to get out of this materials
science story without hearing about carbon nanotubes（纳米碳管）, think again. The MIT team is already in
the lab looking into ways of synthesizing silk-like structures out of materials that are stronger than natural
silk-like carbon nanotubes. Super-silks are on the horizon.
Biologists and engineers are interested in understanding natural silks because they are very light and brittle.
A:Right
B:Wrong
C:Not mentioned
答案：B
解析：
题干的相关内容在第一段第二句，但文中说的是“In a discovery that sounds more like an ancient Chinese proverb than a materials science breakthrough,MIT researchers have discovered that silk gets its strength from its weakness.（一项研究的成果听起来更像一则古代中国谚语而不是材 料科学的突破，在这项研究中，麻省理工学院的研究人员发现，蚕丝的强度源于其脆弱。）”题 干说的是：麻省理工学院的研究员开展这项研究以说明一则古代中国谚语。这与文章所述不 一致。
题干的相关内容在第一段最后一句“Silk gets its extraordinary durability and ductility from an unusual arrangement of hydrogen bonds that are inherently very weak but that work together to create a strong , flexible structure.（蚕丝的异常耐久性和延展性来自一种特别的氢键结构，这些 氢键本质上非常脆弱，但它们共同创造了一种强劲而富有弹性的结构。）”题干说的是：蚕丝的 强韧来自其脆弱的氢键的协同合作。这与文章所述一致。
题干的相关内容在第二段第二句“...natural silks like those produced by spiders have long fascinated both biologists and engineers because of their light weight,ductility and high strength… （……和蜘蛛制造的蛛丝类似的天然蚕丝，因其重量轻，延展性强和韧性高，长期以来令生物学 家和工程师很感兴趣……）”题干说的是：生物学家和工程师对理解天然蚕丝很感兴趣，因为 它们很轻、很脆。这与文章所述不一致。brittle的意思是“脆的，硬但易碎的”。
题干的相关内容在第三段最后一句“...the hydrogen bonds are able to work cooperatively, reinforcing one another against external forces and failing slowly when they do fail,so as not to allowa sudden fracture to spread across a silk structure.（氢键能够齐心协力地合作，相互增援，对抗外 力，同时，它们确实维持不下去时也是慢慢衰退，这样蚕丝的整体结构就不至于出现突然的断 裂。）”题干所说的是：如果氢键因外力而断裂，它们断得很快。这与文章所述不一致。
题干所说的是：麻省理工学院研究组在研究天然蚕丝之前研究过不同的材料。这在文 中没有提及。
题干所说的是：纳米碳管是目前材料学中最盛行的话题。这在文中没有提及。
根据最后一段最后两句可知，研究人员正在想办法用比自然丝状纳米碳管更强韧的材 料合成丝状结构，超级丝就要出现了。题干所说的是：研究表明，比天然蚕丝更强韧的材料有 望在将来出现。这与文章所述一致。第3部分：概括大意与完成句子
未知，从总体中随即抽取容量为25的小样本，此样本的标准差为S。用样本平均数估计总体平均数的置信区间为（ ）
已知时，对总体平均数μ的估计又分为两种情况：①当总体分布为正态时，不论样本n的大小，其标准误都是
。这是样本的方差
在计算中没有用处；②当总体为非正态分布时，只有当样本容量n＞30以上，才能根据样本分布对总体平均数μ进行估计，否则不能进行估计。总体方差
未知时，用样本的无偏方差(
)作为总体方差的估计值，实现对总体平均数μ的估计。因为在总体方差未知时。样本平均数的分布为t分布，故应查t值表。有两种情况：①总体的分布为正态时，可不考虑n的大小；②总体分布为非正态时，只有n>30，才能用概率对其样本分布进行解释，否则不能推论。故本题的正确答案是D。
已知，从总体中随即抽取容量为25的小样本。用样本平均数估计总体平均数的置信区间为（ ）