The resistivity of a material (ρ) is given by:
Where n is the number of charge carriers per unit volume, m is the mass of the carriers, e is the charge and 𝛕 is the time interval between collisions (relaxation time).
Now, number of carriers, n, can change if we fiddle with temperature. Increasing temperature increases thermal generation of carriers. The relaxation time varies inversely with temperature. Mass is constant at all temperatures and for short temperature changes, thermal expansion of solids is ignored. So, option (a) and (b) are correct.
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