题干

假设含有一对同源染色体的精原细胞的一个DNA分子用15N标记,并供给14N的原料。该细胞进行减数分裂产生的4个精子中,含14N的精子所占的比例为                                      (   )

A:0

B:25%

C:50%

D:100%

上一题 下一题 0.0难度 选择题 更新时间:2020-03-25 10:19:53

答案(点此获取答案解析)

D

同类题1

阅读理解

    Computers have beaten human world champions at chess and, earlier this year, the board game Go. So far, though, they have struggled at the card table. So we challenged one AI to a game.

    Why is poker(扑克)so difficult? Chess and Go are “information complete” games where all players can see all the relevant information. In poker, other players' cards are hidden, making it an “information incomplete” game. Players have to guess opponents' hands from their actions----tricky for computers. Poker has become a new benchmark for AI research. Solving poker could lead to many breakthroughs, from cyber security to driverless cars.

    Scientists believe it is only a matter of time before AI once again vanquishes humans, hence our human-machine match comes up in a game of Texas Hold's Em Limit Poker. The AI was developed by Johannes Heinrich, researcher studying machine learning at UCL. It combines two techniques: neural(神经的)networks and reinforcement learning(强化学习).

    Neural networks, to some degree, copy the structure of human brains: their processors are highly interconnected and work at the same time to solve problems. They are good at spotting patterns in huge amounts of data. Reinforcement learning is when a machine, given a task, carries it out, learning from mistakes it makes. In this case, it means playing poker against itself billions of times to get better.

    Mr Heinrich told Sky News: “Today we are presenting a new procedure that has learned in a different way, more similar to how humans learn. In particular, it is able to learn abstract patterns, represented by its neural network, which allow it to deal with new and unseen situations.”

    After two hours of quite defensive play, from the computer at least, we called it a draw.

同类题3

阅读下面的文章,完成小题。

目光里的松阳

彭彭程

    ①“按节下松阳,清江响铙吹。”诗人王维的诗句给深藏在浙西南群山中的松阳注入了一种悠久厚重的历史感。僻远的地理位置,让松阳有幸保存下众多的古村落,也保存了一个良好的生态环境。

    ②这里,蓝天白云是天空的常态,缭绕的云雾是山中的常客;溪水澄碧清亮,茶园舒缓开阔,桂花浓香飘逸。行走于山水间,仿佛置身于一幅立体的水墨长卷中。更为可贵的是,这巨幅山水画中,保留了一百多座格局完整的传统村落。这些山水环绕、林木蓊郁的村落,依据地形的不同,或倚靠青山,或襟带绿水,或俯瞰幽谷,散布在县境各处。

    ③来到村头,或者是一道溪流,溪水汩汩有声,清净见底;或者有一棵甚至几棵高大粗壮的古树,伸展的树冠遮住了一大片地面。再向里走,街巷里大青石铺就的石径弯曲幽深,石径的边沿和墙脚交界处,覆盖着一层湿滑的绿苔;街巷两侧分布着宗祠、水井、水槽、晒谷坛……这些在别处早已经消亡的典型的农村建筑和器具,仿佛一位位耄耋老者,虽历经沧桑却安然无恙。随意推开一扇老旧的门板,走进一座老宅,都会看到曲折的廊道、萦回的天井,地面的方砖大半已经龟裂,纹路纷乱;房屋里外上下,石雕、木雕或彩绘到处可见,构图生动,笔法细腻,堪称精美的艺术品。

    ④古村落弥漫着传统美学的韵味和情致,同时,在种种美的样貌形态背后,还有丰厚的蕴含。石雕、木雕和彩绘,内容多取材于神话传说或传统典籍,八仙过海、岁寒三友、松下问童子、鲤鱼跳龙门……有祝祷的寓意,有教化的作用;“耕读传家”被刻写无数古宅老院的匾额上,并扩展成“耕读传家久,诗书继世长”镌刻于楹柱上,让孩子们在耳濡目染中受到熏陶。杨家堂村是明代开国第一文臣宋濂后裔聚居地,文风昌盛,绵延不衰,这不能不说是先人的文魂引领的结果。古村落中这些传承数千年的文化价值,滋润着一代代人的灵魂。

    ⑤仿佛是上天的特意安排,在遥远宁静的群山之间,安放一种古朴传统的美好,让人们真切地领悟到什么才是诗意的生存。今天,这里的人们也没有辜负上苍的厚爱。

    ⑥西屏街是一条明清老街,长约两公里,青石板的街路两旁,鳞次栉比地排列着下店上宅式的几十家二层木结构店铺。通过设施改建,西屏街既提高了居住舒适度,又较为完好地保存了当年的样子,堪称街区“活态传承”的样本。位于半山腰处的平田村,在古村落改造中强调“原真性保护”,二十八幢老屋被改建成不同档次的民宿,以品位不俗、知名度高吸引着大批的游客。

    ⑦但是,现代化浪潮席卷之处,一应城市乡村都无所逃遁。这样的古村落在不少地方或者被拆除,或者住户被迁走,只留下徒有“古老”外壳、毫无“人气”的所谓的旅游项目。喧嚣和躁动,忙乱和焦虑,速度和效益,织就一张无形巨网,让人们灵性窒息,疲惫不堪。相形之下,这里静谧古雅的氛围、诗意的生活方式和浓郁的人文气息,便愈发显得可贵。

(选自《人民日报》,有删改)