题干

用光学显微镜观察黑藻成熟叶片细胞细胞质的流动时,能观察到叶绿体,但不能观察到染色体的原因主要是

A:黑藻是原核生物,拟核中的DNA没有与蛋白质结合,无染色体

B:黑藻是真核生物,其成熟叶片细胞中已形成染色体,但没有染色

C:黑藻是真核生物,其成熟叶片细胞中不能形成染色体,呈染色质状态

D:叶绿体中有色素分布,染色体没有颜色,染色体被叶绿体中的色素遮掩

上一题 下一题 0.0难度 选择题 更新时间:2015-10-23 06:35:16

答案(点此获取答案解析)

C

同类题4

阅读理解

    Only two countries in the advanced world provide no guarantee for paid leave from work to care for a newborn child. Last spring one of the two, Australia, gave up the bad distinction by setting up paid family leave starting in 2011. I wasn't surprised when this didn't make the news here in the United States — we're now the only wealthy country without such a policy.

    The United States does have one explicit family policy, the Family and Medical Leave Act, passed in 1993. It entitles workers to as much as 12 weeks' unpaid leave for care of a newborn or dealing with a family medical problem. Despite the modesty of the benefit, the Chamber of Commerce and other business groups fought it bitterly, describing it as “government-run personnel management” and a “dangerous precedent (先例).” In fact, every step of the way, as (usually) Democratic leaders have tried to introduce work-family balance measures into the law, business groups have been strongly opposed.

    As Yale law professor Anne Alstott argues, justifying parental support depends on defining(定义) the family as a social good that, in some sense, society must pay for. Parents are burdened in many ways in their lives: there is “no exit” when it comes to children. Society expects — and needs — parents to provide their children with continuity of care. And society expects — and needs — parents to persist in their roles for 18 years, or longer if needed.

    While most parents do this out of love, there are public punishments for not providing care. What parents do, in other words, is of deep concern to the state, for the obvious reason that caring for children is not only morally urgent but important to the future of society. To classify parenting as a personal choice for which there is no collective responsibility is not merely to ignore the social benefits of good parenting; really, it is to steal those benefits because they accrue (累积) to the whole of society as today's children become tomorrow's citizens. In fact, by some estimates, the value of parental investments in children, investments of time and money, is equal to 20-30% of GDP. If these investments bring huge social benefits — as they clearly do — the benefits of providing more social support for the family should be that much clearer.