Make sure the sample is parallel to the magnetic field.
So that only the cross section cuts the magnetic field. This is to ensure the shape of the sample will not have any effect on the external field. So that the x coordinate (field) is purely by the machine.

Question D - magnetic slope. Hysterisis. Slope is positive. Consider why the other graph (hard disk) is negative. One reason is only the hard disk is very thick. That means the saturation ... It is diamagnetic. If only consider saturation, shape is a -x, but other shape is normal. So need to explain why. Shape is not smooth, have some step like thingy. One of the reason is impurities in material or oxidation that gives is a different signal.