单选题

　　[填句补文]Gang Xiao and Ben Schrag at Brown University， visualize the current by measuring subtle（细微的） changes in the magnetic field of an object and ________.

　　A.to shrink it to the size of a desktop computer

　　B.to make it capable of measuring very weak changes in magnetic fields

　　C.as well as revealing microscopic defects in anything from aircraft to banknotes

　　D.converting the information into a color picture showing the density of current at each point