TOEFL Listening: ETS-TOEFL听力机经 - 8OA23E9MC3V8RQ39P$

According to the professor what is one way in which weak magnetic field differ form the strong magnetic fields? A. The weak magnetic field are generated by harden core B. The weak magnetic fields are found only in one particular place C. the strong magnetic fields are of very recent origin D. the strong magnetic fields are confined to rock formation