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问题 选择题

甲、乙、丙三个同种材料制成的正方体，它们的棱长之比为1：2：3，它们的质量分别为3g，24g，36g，它们中有一个是空心的，则空心的正方体是（　　）

A．甲

B．乙

C．丙

D．条件不足，无法判断

答案

甲、乙、丙三个正方体它们的边长分别为1cm，2cm，3cm．

甲的体积V甲=（1cm）3=1cm3；乙的体积V乙=（2cm）3=8cm3；丙的体积V丙=（3cm）3=27cm3；

甲的密度ρ甲=

m甲
V甲
=
3g
1cm3
=3g/cm3；乙的密度ρ乙=
m乙
V乙
=
24g
8cm3
=3g/cm3；丙的密度ρ丙=
m丙
V丙
=
36g
27cm3
≈1.3g/cm3；

不相同的是丙，所以丙是空心的．

故选C．

单项选择题

Air pollution is caused by many things. Great clouds of smoke come from factories. Cars send smoke and fumes into the air. Burning waste fills the air with bits of dirt. Even burning leaves add smoke to the air. Sometimes, planes cannot find their landing spots through smoke. Smoke of all kinds is bad to breathe and can do great harm to the body.
Still, we cannot live without air. We must find a way to clean the air we have.
In time, we may have factories that are run by atomic energy. Our cars may run on smoke-free electric power. Scientists are also working on new ways to help the oil burned by cars from making fumes. Waste may be burned in the sand on ocean floors.
These changes might keep our air clean in the years to come. But until then, many scientists are looking for ways to make air cleaner now.

Air pollution is caused by ______.

A. factories and clouds
B. cars and oil
C. smoke and fumes
D. waste burned on ocean floors
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单项选择题

下列哪项不符合心房颤动的，陆床表现

A．心律绝对不齐
B．心音强弱不一
C．容易听到第四心音
D．心率＞脉率
E．心电图示各导联上P波消失，代之以大小不一、频率350～600/min的f波
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