单选题 The ostrich, the largest bird in the world at present, lives in the drier regions of Africa outside the actual deserts．Because of its very long，powerful legs and the floating effect of its extended wings，it is able to run at great speed over considerable distances．
The female ostrich normally produces about twenty eggs every rainy season． When the female ostrich begins to lay her eggs, however, she does not begin in her own nest．Instead she goes off in search of the nests of neighboring females and laystwo or three eggs in each of them．By the time she has laid eight or nine eggs，she returns and lays the rest in her own nest．
Because of the size of the eggs, the female ostrich cannot lay more than one every two days，so it takes her three weeks to finish laying in her own nest．During that period, she spends a lot of time away from her nest looking for food．And while she is off her nest, other females visit it to lay their eggs amongst hers．By the time she is ready to sit on the eggs to hatch them, there could be up to thirty eggs in her nest, over half of which are not her own．
The female ostrich can comfortably cover only about twenty eggs when she is sitting on the nest, so before settling down she pushes the surplus ten or so eggs out of the nest．The rejected eggs, however, never include any of her own．Each female is remarkably consistent in the size and shape of the eggs she produces, so it is not difficult for her to distinguish her own from those of strangers．
Of all the eggs laid by a colony of ostriches, only a very small number hatch into young birds．There are times when nests are left unprotected, for there are too few males to sit on all the nests at night．Thus there are ample opportunities for their natural enemies to raid the nests and eat the eggs．In fact，nearly 80% of the nests are destroyed．But even if a particular female's nest suffers this fate, there is a good chance that one or two of her eggs will be hatched in the nest of one of her neighbors．

单选题 We learn from the text that an ostrich can go a long distance at high speed as

单选题 Normally,in every rainy season, the female ostrich produces about______．

单选题 The female ostrich would push some of the eggs out of her nest because ______.

单选题 The female os~ich identifies her own eggs by their size and______.

单选题 The female ostrich lays her eggs in her neighbors'nests most probably because______．