[选做题]下列关于放射性元素的半衰期的说法正确的是( ) A.同种放射性元素,在

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问题:

[选做题]下列关于放射性元素的半衰期的说法正确的是(  )

A.同种放射性元素,在化合物中的半衰期与在单质中相同,放射性元素的半衰期与元素所处的物理和化学状态无关

B.质量为m的铀经过两个半衰期后原来所含的铀元素的原子核有

1
4
m发生了衰变

C.氡的半衰期是3.8天,若有4g氡原子核,则经过7.6天就只剩下1g氡原子核

D.氡的半衰期是3.8天,若有4个氡原子核,则经过7.6天就只剩下一个氡原子核

考点:α衰变β衰变半衰期
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法律是我们的“保护伞”和“守护人”。因为法律:

①维护公共秩序   ②维护正常的经济秩序

③维护公共安全   ④满足人们的一切需要

A.①②③

B.①②④

C.①③④

D.②③④

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阅读短文,从方框中选择适当的动词,并用其适当形式填空。有的需要加助动词或不定式符号 (每词限用一次)。

Mr. Jackson is sixty-four years old. He (1) ______________ in a high school in a small town. All the town people know him very well because he (2) ______________ a teacher there for over forty years.

Mr. Jackson was born in London. He was twenty-two when he (3) ______________ college. Then he came to the small town to teach. He is hard-working and always makes his lessons interesting. He likes his students and the students like him, too. He (4) ______________ as the best teacher in the school.

Mr Jackson will retire next year. But he doesn’t plan (5) ______________ back to London. “I think this small town is my hometown. I won’t leave it,” he said.

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罗茨风机启动时,先启动罗茨风机,再打开放空阀。

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人力资源计划的制定工作应该由人力资源管理部门全权负责,企业其它部门与此无关

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女,20岁,系统性红斑狼疮患者,狼疮肾,尿蛋白持续++,足量糖皮质激素治疗4周无效,应()

A.加大激素用量

B.雷公藤

C.加抗疟药

D.加用免疫抑制剂

E.加利尿剂

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