有两个关系R、S，如下： 由关系R通过运算得到关系S，所使用的运算为A) 选择

多数食品容易吸收空气中的水分变潮湿，并与空气中的氧气反应而腐败。生产上多在食品中放入一小包CaO粉末，可使食品保持干燥。现已研究成功在食品中放入一小包铁粉（包裹在多孔泡沫中），铁粉吸收水分和氧气变为铁锈，从而保护食品。下列说法不正确的是

A．生成的铁锈是一种混合物

B．铁粉干燥剂能与水和氧气发生化学反应

C．也可选择浓硫酸作食品干燥剂

D．每种干燥剂袋上都要有明显的“勿食”字样

麦蛾的头式是（）。

A、前口式

B、后口式

C、下口式

D、上口式

We learned that fossil fuels were formed before and during the time of the dinosaurs（恐龙）. We are using up the fuels. We can save fossil fuels by protecting and finding ways to use energy from “endless sources”, like the sun and the wind. Some people suggest that we should start using hydrogen（氢）.

Hydrogen is a colorless, smell-less gas that takes up 75 percent of the whole universe. Hydrogen is found on earth only in mixture with other elements such as oxygen, carbon and nitrogen. To use hydrogen, we must separate it from these other elements.

Hydrogen can be used in NASA’s space program as fuel for the spaceships, and in fuel cells that provide heat, electricity and drinking water for astronauts. Fuel cells are devices that directly change hydrogen into electricity. In the future, hydrogen could be used to fuel vehicles and aircraft and provide power for our homes and offices.

Hydrogen can be made from natural gas. An electrical current can also be used to separate water into its elements of oxygen and hydrogen. Some plants in the sea and bacteria（细菌） give off hydrogen through sunlight under certain conditions.

Hydrogen as a fuel is high in energy. Yet a machine that burns pure hydrogen produces almost zero pollution. NASA has used liquid hydrogen since the 1970s to drive rockets and spacecraft into orbit. Hydrogen fuel cells power the spacecraft’s electrical systems, producing clean by-product-pure water, which the crew drinks.

56. We can get hydrogen from the following EXCEPT ______ according to the passage.

A. natural gas    B. water                    C. creature in the sea         D. bacteria

57. Why could the hydrogen be used widely in the future?

A. Because it is of high quality.               

B. Because it has no pollution. 

C. Because it is easy to get it from all kinds of ways.      

D. Because there are much hydrogen existing in the universe.

58. The passage mainly tells us ________.

A. the usage of a new energy source        B. the future energy sources

C. a new energy source －hydrogen       D. how to get the energy source

什么是设备对地电压？

王先生打算退休后，每季度从银行取出9000元维持生计；若年实际利率为8.24%，则王先生现在应该存入( )元。

A．485400
B．450000
C．121400
D．400000