下列变化中,属于化学变化的是() A.干冰升华 B.灯管发亮 C.铁丝弯曲 D.

题型:选择题

问题:

下列变化中,属于化学变化的是(  )

A.干冰升华

B.灯管发亮

C.铁丝弯曲

D.酒精燃烧

考点:物理变化和化学变化的特征和判别物理性质和化学性质的区别和应用氨气的性质二氧化硫的性质
题型:选择题

可摘局部义齿热处理后发现基托中有气泡,制作过程中可能原因有 ()

A.充胶时机过早

B.粉液比例不当

C.热处理升温过快

D.装盒时石膏有倒凹

E.充胶时压力不足

题型:选择题

直肠痛切除术能否保留肛门,主要取决于

A.肿瘤距肛门的距离

B.肿瘤的病理类型

C.肿瘤是否已侵犯肠管周围

D.肿瘤有无远处转移

E.左半结肠的长度

题型:选择题


The central problem of economics is to satisfy the people’s and nation’s wants. The problem we are faced with is that our (1) , here identified as money, are (2) . The only way we can resolve our (3) is to make choices. After looking at our resources, we must examine our list of (4) and identify the things we need (5) , those we can postpone, and (6) we cannot afford. As individuals, we face the central problem involved in economics — deciding just how to allocate our limited resources to provide ourselves with the greatest (7) of our wants.
Nations face the same problem. As a country’s population (8) , the need for more goods and services grows (9) . Resources necessary to production may increase, but there are (10) enough resources to satisfy the total desires of a (11) . Whether the budget meeting is taking place in the family living-room, in the conference room of the corporation (12) of directors, or in the chamber of the House of Representatives in Washington, the basic problem still exists. We need to find (13) of allocating limited resources in order to satisfy unlimited wants.
A short time ago, economists divided goods into two categories, free and economic. The former, like air and water, were in (14) abundance that economists had no concern about (15) of scarcity and what to do about it. Today many of these "free goods" are in (16) very expensive to use. Pollution has made clean air and water expensive for producers who have to filter their waste products, for consumers who ultimately pay for the producers’ extra costs, and (17) tax-payers who pay for the government’s involvement in cleaning the environment.
In the 1990s, almost all goods are (18) . Only by effort and money can they be (19) in the form people wish.
Meeting the needs of people and the demands from resources available leads to the basic activity of production. In trying to (20) unlimited wants from limited economic goods, production leads to new problems in economics.

题型:选择题

番茄幼苗在缺镁的培养液中培养一段时间后,与对照组相比,其叶片光合作用强度下降,原因是( )

A.光反应强度升高,暗反应强度降低
B.光反应强度降低,暗反应强度降低
C.光反应强度不变,暗反应强度降低
D.光反应强度降低,暗反应强度不变

题型:选择题

某汽车加油站2个单罐容积为30m3的煤气罐,1个单罐容积为50m3柴油罐。该加油站的等级是()。

A.一级

B.二级

C.四级

D.三级

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