题目内容：
回答题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 great surprise, that the big bird had swallowed a bunch of diamonds．How could such a strange thing happen?
Like many other birds, the ostrich swallows small stones that stay inside its " gizzard．" The gizzard is a bird's second stomach．It is where the food is ground up．The small stones help to grind up the food so it can be digested．They do the chewing, because birds don't have teeth．In the case of the ostrich with the 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 around, it willeat just about anything．Sadly for ostriches in zoos this can be a fatal habit．The tendency to swallow anything it sees has caused the death of many an ostrich．Cruel or careless people often throw things into the bird's living space．They throw keys, coins, even large objects such as horseshoes．The ostrich swallows them without hesitation．Coins can be the worst．Inside the ostrich they wear down to a razor sharp edge．They will cut open the bird's gizzard from the inside．One young zoo ostrich died with 484 coins, weighing more than eight pounds, in its gizzard．
Ostriches eat stones because they don't have________ A．enough food
B．bird seed
C．teeth
D．diamonds
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