In thicker materials the particles can move more easily, therefore the resistance has to decrease.
Hence Option (b) is correct.
Resistivity, which is sometimes represented by the Greek letter rho, is quantitatively equal to a specimen's resistance R times its cross-sectional area A times its length.
ρ = R × A/ l
R = ρ × l/ A
It means the Resistance of the material is inversely proportional to the area of the material.
As the area of the material increases, the resistance decreases and vice versa.
Hence, In thicker particles, there is a greater area available and that's why resistance has to decrease so that particles can move more easily.
Hence, Option (b) is correct.
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