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问题 单项选择题

非重瓣类的桃花品种是（）

A.碧桃

B.白桃

C.绛桃

D.绯桃

答案

参考答案：B

计算题

水平放置的两个平行金属轨道相距0.2m，上面有一质量为0.04kg的不计内阻的均匀金属棒ab，电源电动势为6V，内阻为0.5Ω，滑动变阻器调到2.5Ω时，要在金属棒所在位置施加一个垂直于ab的匀强磁场，才能使金属棒ab对轨道的压力恰好为零。(g=10m/s2)

(1)求匀强磁场的大小和方向．

(2)若保持磁感应强度B的大小不变，方向顺时针转37°，此时ab仍然静止，则轨道对ab的支持力和摩擦力为多大？
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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.

In the third paragraph, the word "striking" means ______.

A．unusual

B．simple

C．weak

D．slow
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