[单项选择]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
A. color
B. number
C. shape
D. weight