gwd30-20 google到一半答案，是A

我尝试google了一下（key words: the magnetic fields emitted  security surveillance system)

http://www.google.com/search?hl=zh-CN&q=the+magnetic+fields+emitted+security+surveillance+system&btnG=Google+%E6%90%9C%E7%B4%A2&lr=

请看第5条：

news@nature.com - hop'til you drop

Michael Mclvor of the Heart Institute of St Petersburg, Florida, and colleagues announce in the New England Journal Medicine that the magnetic fields emitted by one type of shop security surveillance system, of which there are 200,...