问题 选择题

如图所示,a、b、c、d是某匀强电场中的四个点,它们正好是一个矩形的四个顶点,ab=cd=10cm,ad=bc=20cm,电场线与矩形所在平面平行.已知a点电势为20V,b点电势为24V,则(  )

A.场强大小一定为E=40V/m

B.cd间电势差一定为4 V

C.场强的方向一定由b指向a

D.c点电势可能比d点低

答案

A、C根据题设条件可知,场强方向无法确定,知道了ab间电势差,但不能根据E=

U
d
求出场强的大小.故AC错误.

B、根据匀强电场的电场线与等势线分布的特点可知,cd间电势差必等于ba间电势差,为4V.故B正确.

D、cd间电势差为4V,则知c点电势比d点高4V.故D错误.

故选B

阅读理解

Cancer researchers urged people on Wednesday to take more vitamin D to lower risk of cancer, saying studies showed a clear link. "Our suggestion is for people to increase their intake, through diet or a vitamin supplement," Dr. Cedric Garland said in a telephone interview.

Garland's research team reviewed 63 studies, including several large long-term ones, on the relationship between vitamin D and certain types of cancer worldwide between 1966 and 2004. "There's nothing that has this ability to prevent cancer," he said, urging governments and public health officials to do more to fortify foods with vitamin D.Garland is part of a University of California at San Diego Moores Cancer Center team that published its findings this week online in the American Journal of Public Health.    Vitamin D is found in milk, as well as in some fortified orange juice, yogurt and cheeses, usually at around 100 international units (IU) a serving. People might want to consider a vitamin supplement to raise their intake to 1000 IUs per day, Garland said, adding that it was well within the safety guidelines established by the National Academy of Sciences.

The authors said that taking more vitamin D could be especially important for people living in northern areas, which receive less vitamin D from sunshine.

African Americans, who don't produce as much of the vitamin because of their skin colour, could also benefit significantly from a higher intake, the authors said.

小题1: According to the passage, people are advised to take more Vitamin D because_________.

A.it is nutritious

B.it can‘t harm people’s health

C.it is not taken enough every day

D.it can lower cancer risk小题2: Which of the following can not help people get more Vitamin D?

A.Have more meat

B.Have some sunshine

C.Have a vitamin supplement

D.Have more fortified cheese小题3:Who can Garland probably be?

A.A scientist

B.A health researcher

C.A doctor

D.A public health official小题4:Which of the following food can lower people‘s chance of getting cancer?

A.Fortified orange juice

B.Milk

C.Fortified yogurt

D.All of the above小题5:People from which area should take more Vitamin D according to the passage?

A.Asian people

B.American people

C.European people

D.African people

单项选择题