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