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1990年，加拿大多伦多大学的米切尔·洛林宣布：在2.6亿年以前，栖息在美国得克萨斯山区一种外形像蜥蜴的名叫四角龙的爬行动物，确实是哺乳动物的远古“______”，①填补了进化链中，从爬行动物到哺乳动物中缺少的一环。1987年，米切尔·洛林研究了一块盘龙类的头骨化石。随着研究的深入，化石上的一些细节却使他困惑不解。②大多数的盘龙类在腭部有很大的孔，而在较进化的兽孔类身上，这个孔已被封闭，四角龙也有一个腭孔，但已明显缩小，其直径仅为0.635厘米。③盘龙类在头部背面有一块很大的骨，用以支持颌骨，在兽孔类中，这块骨头已大大缩小了，而四角龙的这块骨要较兽孔类大，又较盘龙类小。更为重要的是，四角龙的头角上有个骨架，穿过颞孔的咀嚼肌像兽孔类那样直接依附其上，而不像盘龙类那样由肌腱相接。这些发现使洛林相信，四角龙是盘龙类和兽孔类之间的一个过渡类型。他又把从几块盘龙类和兽孔类动物化石中获得的信息输入电脑(包括腭孔、颞孔形状、头颅骨形状、牙齿数量和着生位置等)，④由电脑判断出两者之间的联系。结果表明，在进化树上，通向兽孔类一边的第一个分叉就是四角龙。

文中①～④处应填入的词是（）。

A．于是/由于/因此/随后

B．从而/因为/于是/然后

C．随着/如果/此外/于是

D．从而/因为/此外/然后

答案

参考答案：D

多项选择题

局域网络在目前常用的有哪几种（）

A.以态网

B.令牌环网

C.FDDI

D.令牌总线
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填空题

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.
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