Dalton wondered why the heavier and lighter gases in the atmosphere did not separate as oil and water do.He finally concluded that the constituent 1 must exist in the form of 2 particles or atoms and that these 3 be completely mixed together in the 4 .This threw a new light in 5 laws of definite proportions.It was 6 necessary to suppose that the atoms 7 combinesintossmall groups of uniform 8 and so form more complex substance; 9 the mystery of this law was 10 .Dalton suggested, for example, that carbon monoxide 11 formed bythe one with one 12 of atoms of carbon and oxygen, 13 carbon dioxide results from a single 14 of carbon uniting with two atoms 15 oxygen.Assuming this to be true, 16 the definite proportions of Prout's law (17 all chemical compounds the different constituents 18 enter in unvarying proportions) would naturally 19 the relative weights of the many 20 kinds of atoms.

1.A.gases B.pieces C.things D.gas

2.A.timely B.trimmed C.tiny D.topical

3.A.ought B.could C.will D.must

4.A.sky B.atmosphere C.oxygen D.environment

5.A.various B.many C.the D.those

6.A.only B.too C.that D.extremely

7.A.should B.were C.get D.could

8.A.construct B.structure C.piece D.feature

9.A.but B.also C.thus D.nevertheless

10.A.solved B.discovered C.broked D.told

11.A.must B.been C.were D.is

12.A.putting B.pairing C.placing D.preparing

13.A.while B.when C.therefore D.also

14.A.piece B.type C.atom D.measure

15.A.from B.in C.of D.for

16.A.then B.so C.by D.that

17.A.from B.in C.to D.for

18.A.are B.wouldn’t C.always D.never

19.A.describe B.unit C.mix D.reveal

20.A.different B.usual C.important D.chemical

答案+解说：
1.
【答案】A
【解析】根据上文中的heavier and lighter gases可以确定,本空中应填入“组成的气体”，即constituent gases。
2.
【答案】C
【解析】根据句子意思判断,句中的词意为微粒的意思，选项C.tiny合乎题意。
3.
【答案】D
【解析】情态动词must在这里是“必定、必须”的意思。
4.
【答案】B
【解析】sky意为“天空”，atmosphere意为“大气”, oxygen意为“氧气”, environment意为“环境”,只有B项正确。
5.
【答案】C
【解析】这帮助解释了大气组成气体的比例确定原理。因为是特指这一原理，用定冠词the。
6.
【答案】A
【解析】only necessary的意思是“仅仅是必要”。故A正确。
7.
【答案】D
【解析】根据句子意思判断, could是“能够”的意思。
8.
【答案】B
【解析】根据句子意思判断,这里需要“结构”这个词，而B合乎题意。
9.
【答案】C
【解析】but，nevertheless表示转折意为“但是”，“然而”；thus意为“因此”，如There has been no rain.Thus, the crops are likely to suffer.一直没有下雨，因此庄稼可能会受害。这里与上文有因果关系，故应选C。
10.
【答案】A
【解析】solve相当于find a solution or an answer to,意为“解决”、“解开”,如: solve a problem (a puzzle, a mystery)；discover相当于find something existing but not known before,意为“发现”,如: discover a place (a fact, the truth)。因此应选A。
11.
【答案】D
【解析】因为是表一般规律，又是单数，所以用is这个词。
12.
【答案】B
【解析】Pair这里是“匹配，配成双”的意思。
13.
【答案】A
【解析】while表示对比。如：Some substances are soluble, while others are not.一些物质是可溶的，而另一些物质是不可溶的。
14.
【答案】C
【解析】根据句子意思判断, atom正确。
15.
【答案】C
【解析】of表示“含量，分量”。
16.
【答案】A
【解析】then的意思是next, after that。
17.
【答案】D
【解析】for的意思是“对于，就……而言”。
18.
【答案】C
【解析】根据句子意思判断。
19.
【答案】B
【解析】describe意为“描述”；unite意为“结合，合并”；mix意为“(不同的东西)混合在一起”，如：mix flour and water；reveal意为“揭露，揭示”。
20.
【答案】A
【解析】根据句子意思判断.