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问题 单项选择题

以下数据结构中，属于线性数据结构的是

A．集合

B．线性表

C．二叉树

D．图

答案

参考答案：B

解析： 所谓的线性结构：如果一个非空的数据结构满足下列两个条件，即 1) 有且只有一个根结点； 2) 每一个结点最多有一个前件，也最多有一个后件。同时满足两个条件的只有线性表，而其他三种数据结构的结点可能存在多个前件或后件，所以不是线性结构。

多项选择题

轻骨料混凝土与同等级普通混凝土相比，其特点主要表现在( )。

A．表观密度小

B．耐久性明显改善

C．弹性模量小

D．节约能源

E．导热系数大
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单项选择题

(五)


Space travel is definitely bad for astronauts’ bones, reducing their bone density after only a month of weightlessness, according to French research published on Friday.
Laurence Vico and his fellow workers at St Etienne University called for more research into the effects of microgravity, after their study of 15 astronauts from the Russian MIR station showed bone loss continued throughout space flights.
"Bone loss was especially striking in four astronauts, " the scientists reported in the Lancet Medical Journal.
They measured the Bone Mineral Density(BMD) of bones in the forearm and lower leg of the astronauts who had spent one to six months in space.
The BMD loss was significant in the tibia of the lower leg, a weight-bearing bone, but barely changed in the radius of the forearm. "Our results indicate the need to investigate not only different bones, but also different areas of the same bone since not all sites of the skeleton are similarly affected by space conditions, " they added.
Without gravity the body isn’t bearing any weight so there is no need for calcium which makes bones p, and it becomes empty into the bloodstream.
The research team suggested in future scientists should try to determine if the loss of bone density was only on weight-bearing bones on longer flights, also the possible recovery after returning to Earth.

Scientists have found that ______ .

A．the BMD loss may cause serious illness to astronauts

B．the BMI) loss may cause some change in astronauts’ bodies

C．astronauts shouldn’t care about the BMD loss

D．astronauts should take some calcium before space travel
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