Which one of the following statements regarding Ohm's law is not correct?

• A. Ohm's law is an assumption that current through a conductor is always directly proportional to the potential difference applied to it.
• B. A conducting device obeys Ohm's law when the resistance of a device is independent of magnitude and polarity of applied potential difference.
• C. A conducting material obeys Ohm's law when the resistance of material is independent of the magnitude and direction of applied electric field.
• D. All homogeneous materials obey Ohm's law irrespective of whether the field is within range or strong.