单项选择题

Text 4 Each suburban housewife,wrote Betty Friedan in 1963,struggles with a single quesLion as she makes the beds,shops for groceries,drives children to school and lies beside her husband at night:"Is this all?"A few years after her ground-breaking book The Feminine Mystique was published,the Cen8us Bureau began collecting data on the proportion of mothers who opt to stay at home.Over the subsequent decades the statistics answered Friedan's question with a heartfelt no.In 1967 the share of mothers who did not work ouLside the home stood at 4996;by the turn of the millennium it had dropped to just 23%.Many Lhought this number would continue to fall as women sought to"have it all".Instead,the proportion of stay-at-home moLhers has been rising steadily for the past 15 years,according to new data gathered by the Pew Research Centre.This partly reflects demographic change.Immigrants,a rising share of Lhe relevant generation,are more likely to be stay-at-home mums than women born in America.There is an economic component to the change,too:at the end of the 1990s,when mothers staying at home were at Lheir rarest,the economy was creating so many jobs that most people who wanted work could find it.Now more report that they are unable to do so,or are studying in the hope of finding work later.But there is also an element of choice:a quarter of stay-at-home mothers have coUege degrees.Taken as a whole,the group includes mothers at both ends of the social scale.Some are highly educated bankers'wives who choose not to work because they don't need the money and would rather spend their time hot-housing their toddlers so that they may one day get into Harvard.Others are poorer but calculate that,after paying for child care,the money they make sweeping filoors or serving burgers does not justify the time away from their little ones.he first group is fturly small.Pew estimates that there are 370,000 highly educated and afflu-ent slay-at-home mothers(defined as married mothers with children under 18 who have at least a master's degree and family income in excess of$75,000).That is 5%of all stay-at-home mothers with working husbands.One third of stay-at-home moLhers are single or cohabiting,and on average they are poorer than the rest.
Betty Friedan is mentioned in the first paragraph to______

A.discussaboutfemalewriters
B.showtheauthor'sconLribution
C.introducethetopicofhousewives
D.describethelifeofsinglemothers
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单项选择题
光是生命活动所必需的刺激信号,而不仅是能量的来源。由于光的作用,人和许多动物才能感觉到自然界的千姿百态和绚丽多彩,才能具有空间定向与时间定位本领,从而与生存环境保持统一。光作为刺激信号与生命发生的最为密切的关系,莫过于视觉的形成。人从外界环境获得的信息有80%以上来自眼睛。但是,只有眼睛还不能形成视觉,必须有光作为刺激信号并与眼睛接收与加工光信号的机构相配合才能形成视觉。视网膜上的光感受细胞对光产生反应,把光所携带的景物信息转换为神经电信号,经加工后传到大脑,再经脑的综合分析产生视觉,这才能看到环境景物。光运动也是光作为一种信号对生命活动发生作用的一个重要方面。所谓光运动,就是那些由光调节的、生物体的整体或某一部分发生空间位移的行为反应。光运动使生物体具有了空间定向本领。多细胞植物虽然不能有来去自由的运动,但它们可表现出弯曲、扭转等运动现象,而这些运动往往受着光照的控制。若光的作用只是触发这类运动,则称为感光性。要是光的方向决定了运动方向,就称为向光性。具有光合作用的植物,向光性突出表现在把叶面转向太阳光的入射方向。生物对于光的刺激产生定向运动,称为趋光性。这些生物要在光源与背景之间,不同强度的光源之间选择运动方向。朝向光刺激的运动反应叫正趋光性,背向光刺激的运动反应叫负趋光性。地球的自转和公转,产生了昼夜和季节的变化。生活在地球上的生物,是在这种光照的周期性变化环境中进化而来的,因而适应了环境,产生了生命活动的内在节奏性。生物体内与环境的周期变化相对应的周期性变动,称为生物节律,它是生物体对环境周期性变化的反应。造成生物节律现象并使之得以维持的内部机制称为生物钟,生物钟的存在使生物体具有了时间定位本领。一个具有某种生物钟的动物,如果被迅速转移到环境周期不再与运动节律同步的地点,则生物钟还能按与原来环境周期同步的节律继续作用一段时间。对第四段中“生物钟”的理解,正确的一项是( )
A.生物钟与生物的空间定向有关
B.生物钟与光照的变化毫无关系
C.生物钟不因环境的变化而变化
D.生物钟是生物的内在节律系统