如图上小下大的台形容器，装有一定量的水银．当水银受热膨胀时，若不计容器的热膨胀，水银对容器底部的压强将如何变化（　　）

A．变大

B．变小

C．不变

D．都有可能

Questions 6-10 are based on Passage Two:Passage TwoEducating girls quite possibly yields a higher rate of return than any other investment available in the developing world. Women’s education may be unusual territory for economists, but enhancing women’s contribution to development is actually as much an economic as a social issue. And economics, with its emphasis on incentives, provides guideposts that point to an explanation for why so many girls are deprived of an education.Parents in low-income countries fail to invest in their daughters because they do not expect them to make an economic contribution to the family: girls grow up only to marry into somebody else’s family and bear children. Girls are thus seen as less valuable than boys and kept at home to do housework while their brothers are sent to school- the prophecy becomes self-fulfilling, trapping women in a vicious circle of neglect.An educated mother, on the other hand, has greater earning abilities outside the home and faces an entirely different set of choices. She is likely to have fewer but healthier children and can insist on the development of all her children, ensuring that her daughters are given a fair chance. The education of her daughters then makes it much more likely that the next generation of girls, as well as of boys, will be educated and healthy. The vicious circle is thus transformed into a virtuous circle.Few will dispute that educating women has great social benefits. But it has enormous economic advantages as well. Most obviously, there is the direct effect of education on the wages of female workers. Wages rise by lo t0 20 percent for each additional year of schooling. Such big returns are impressive by the standard of other available investments, but they are just the beginning. Educating women also has a significant impact on health practices, including family planning.

The passage mainly discusses ().

A．unequal treatment of boys and girls in developing countries

B．the potential earning power of well-educated women

C．the major contributions of educated women to society

D．the economic and social benefits of educating women