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