问题 单项选择题

在“学生-选课-课程”数据库中如下三个关系: S(S#,SNAME,SEX,AGE),SC(S#,C#,GRADE),C(C#,CNAME,TEACHER)则查找学生号为“20040901”的学生的“计算机”课程的成绩,至少将使用关系()。

A.S和SC

B.SC和C

C.S和C

D.S、SC和C

答案

参考答案:B

解析:在若干个表中进行查询时,根据所需要查出的屑性来确定查询的表的范围。注意:在C表中找课程的C#,然后在sc表中找到grade,故B)选项正确;而D)选项的s表示多余的。

阅读理解

"Have you ever been out on a boat and felt it lifted up by a wave? Or have you jumped in the water and felt the rush of energy as waves came over you?" asked Jamie Taylor of the Wave Energy Group at the University of Edinburgh. "There is certainly a lot of energy in waves," he said.

Scientists are working to use that energy to make electricity. Most waves are created when winds blow across the ocean. "The wind starts out by making little ripples (涟漪) , but if they keep on blowing, those ripples get bigger and bigger and turn into waves," Taylor said. " Waves are one of nature 's ways of picking up energy and then sending it off on a journey.

When waves come toward the shore, people can set up dams to block the water and send it through a large wheel called a turbine (涡轮机) . The turbine can then power an electrical generator to produce electricity.

"The resource is huge," said Janet Swain of the Worldwatch Institute. "We will never run out of wave power. " Besides, wave energy does not create the same pollution as other energy sources, such as oil or coal. Oceans cover three-quarters of the Earth's surface - that would make wave power seem ideal for creating energy throughout the world, though there are some weak points yet to overcome.

Swain said that wave power still costs too much money. She also said that its effects on sea animals are still unknown. What is more, wave power could affect fishing and boat traffic.

Traditional sources of energy like oil and gas may someday run out. " Demand for energy to power our TVs and computers, drive our cars, and heat and cool our homes is rising rapidly throughout the world," Swain said. In the future when you turn on a light, an ocean wave could be providing the electricity!

64. The writer uses the two questions at the beginning of the passage to _______.

A. test the readers' knowledge about waves          B. draw the readers' attention to the topic

C. show Jamie Taylor's importance                            D. invite the readers to answer them

65. The underlined phrase "picking up" (paragraph 2) is closest in meaning to _______.

A. starting again            B. speeding up              C. gathering           D. improving

66. We can make better use of wave energy if we _______.

A. shorten its journey to thousands of homes       B. build more small power stations on the oceans

C. reduce the cost of turning it into electric power

D. quicken the steps of producing electricity

67. It can be inferred that some day we might not worry about ________.

A. air pollution             B. our boat traffic         C. our power supply             D. our supply of sea fish

名词解释