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 in zoos 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．
第 47 题 Ostriches eat stones because they don’t have ．