A great deal can be learned from the actual traces of ancient human locomotion： the footprints of early hominids. The best-known specimens are the remarkable tracks discovered at Lactoli， Tanzania， by Mary Leaky. These were left by small hominids around 3.6 to 3.75 million years ago， according to potassium - argon dates of the volcanic rocks above and below this level. These hominids walked across a stretch of moist volcanic ash， which was subsequently turned to mud by rain， and which then set like concrete.

　　Examination of he shape of the prints revealed to Mary Leakey that the feet had a raised arch， a rounded heel， a pronounced ball， and a big toe that pointed forward. These features， together with the weight-bearing pressure patterns， resembled the prints of upright-walking modern humans. The pressures exerted along the foot， together with the length of stride， which averaged 87 centimeters， indicated that the hominids had been walking slowly. In short， all the detectable morphological features implied that the feet that left the footprints were very little different from those of contemporary humans.

　　A detailed study has been made of the prints using photogrammetry， a technique for obtaining measurements through photographs， which created a drawing showing all the curves and contours of the prints. The result emphasized that there were at least seven points of similarity with modern bipedal prints， such as the depth of the heel impression， and the deep imprint of the big toe. M Day and E. Wickens also took stereophotographs of the Lactoli prints and compared them with modern prints make by men and women in similar soil conditions. Once again， the results furnished possible evidence of bipedalism. Footprints thus provide us not merely with rare impressions of the soft tissue of early hominids， but also with evidence of upright waling that in many ways is clearer than can be obtained from the analysis of bones.

　　The study of fossil footprints is not restricted to examples from such remote periods. Hundreds of prints are known， for example， in French caves dating from the end of the last ices age， approximately 10，000 years ago. Research by Leon Pales， using detailed silicon resin molds of footprints mostly made by bare feet， has provided information about this period.

　　20.What does the passage mainly discuss？

　　（A）The analysis of footprint fossils

　　（B）Accurate dating of hominid remains

　　（C） the career of Mary Leakey

　　（D）Behavioral patterns of early humans

　　21.The word "remarkable" in line 2 is closest in meaning to

　　（A） extraordinary

　　（B） enormous

　　（C） various

　　（D） orderly

　　22.The age of the Laetoli footprints was estimated by

　　（A） testing the fossilized bones of the hominids

　　（B） studying the shape of the footprints

　　（C） analyzing nearly rock layers

　　（D） comparison with footprints from other locations

　　23.It can be inferred that the footprints in volcanic ash at Laetoli were well preserved because

　　（A） they were buried by a second volcanic eruption

　　（B） the ash contained potassium anti argon

　　（C） the ash was still warm from the volcanic cruptions

　　（D） suitable conditions caused the ash 10 harden

　　24.Which or the following is NOT mentioned as a characteristic of the feet in Mary Leakey s fossil find？

　　（A）The shape or the heel

　　（B）The number of toes

　　（C）A raised arch

　　（D）A pronounced ball

　　25.The word "exerted" in line 11 is closest in meaning to

　　（A） influenced

　　（B） applied

　　（C） returned

　　（D） lessened

　　26.The figure of 87 centimeters mentioned in line 1 2 refers to the size of the

　　（A） objects carried by the hominids

　　（B） steps taken by the hominids

　　（C） hominids bodies

　　（D） hominids feet

　　27.Why does the author mention the "heel impression" in line 19？

　　（A） To emphasize the size of the hominids foot

　　（B） To speculate on a possible injury the hominid had suffered

　　（C） To give an example of similarity to modern human footprints

　　（D） To indicate the weight of early hominids

　　28.The word "restricted" in line 26 is closest in meaning to

　　（A） limited

　　（B） improved

　　（C） continued

　　（D） succeeded

　　29.What can be inferred about the footprints found in French caves mentioned in the last paragraph？

　　（A）They show more detail than the Laetoli prints.

　　（B）They are of more recent origin than the Laetoli prints.

　　（C）They are not as informative as the Laetoli prints.

　　（D）They are more difficult to study than the Laetoli prints

　　30.Which of the following terms is defined in the passage？

　　（A） "hominids" （line 3）

　　（B） "arch" （line 9）

　　（C） "photogrammetry" （line 16）

　　（D） "silicon resin molds"（line 29）