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45、The best title for this short passage is ．


46、How do you know which bus to take if one goes somewhere?


47、根据以下材料回答第 1～4 题：
Passage Four
You know that pearls grow inside oysters，but would you ever think to look for diamonds inside an ostrich?Well，a hunter once shot an ostrich and discovered，to his greatsurprise，that the big bird had swallowed a bunch of diamonds．How could such a strangething happen?
Like many other birds，the ostrich swallows sinail stones that stay insied its“giz—zard．”The gizzard is a bird’s second stomach．It is where the food is ground up．Thesmall stones help to grind up the food so it can be digested．They do the chewing，becausebirds don’t have teeth．In the ease of the ostrich with diamonds，the bird simply had expensive taste in rocks．He used the diamonds to help digest his dinner．
Diamonds and stones aren’t all that an ostrich will swallow．If there are no stones a-round，it will eat just about anytin9．Sadly for ostriches inzoos this call be a fatal habit．
The tendecy to swallow anything it sees has caused the death of many an ostrich．Cruel orcareless people often throw things into the bird’s living space。They ttlron keys，coins，e-ven large obj ects such as horseshoes．The ostrich swallows them with out hesitation．Coins can be the worset．Inside the ostrich they wear down to a razor sharp edge．They will cut open the bird’s gizzard from the indide．One young zoo ostrich died with 484coins，weighing more than eight pounds，in its gizzard．
Ostriches eat stones because they don’t have ．



48、The ostrich is not srrtart enough to ．


49、The phrase“they do the chewin9”makes the rocks seem as though they are．


50、“Fatal”is another word for ．


51、根据以下材料回答第 1～4 题：
　　　　　　　　　　　 　　　　　　　　Passage Five
Although no one is certain why migration Occurs，there are several theories．One the-ory is based upon the premise that prehistoric birds of the northen Hemisphere were forced south during the Ice Age，when glaciers covered large parts of Europe，Asia，and North America．As the glaciers melted，the birds came back to their homelands，spend the sum-mer，and then went south again as the ice advanced in winter．In time，the migration be-came a habit，and now，although the glaciers have disappeared，the habit continues。
Another theory proposed that the ancestral home of all modern birds was the tropics．When the region became overpopulated，many species were crowded north．During thesummer，there was plenty of food，but during the winter，scarcity forced them to returnto the tropics．
A more recent theory，known as photoperiodism，suggests a relationship between increasing daylight and the stimulation of certain glands in the birds’bodies that may preparethem for migration．One scientist has been able to cause midwinter migrations by exposingbirds to artificial periods of daylight．He has concluded that changes occur in the bodies ofbirds due to seasonal changes in the length of daylight．
According to one theory，when the glaciers disappeared，birds　　 　．



52、The author states that birds left the tropics because　　　 ．


53、Why did one scientist expose birds to artificial daylight?


54、According to the theory of photoperiodism，　　　　．