If there are two types of collision mechanisms in a conducting material, and each mechanism has its own collision time, τ1 and τ2, respectively, what is the overall collision time τ (in terms of τ1 and τ2? (b) assume each collision mechanism independently leads to its own drude's resistivity, say ρ(1) and ρ(2), then what is the total resistivity ρ? (c) what is the overall mobility μ in terms of μ(l) and a2), the mobility due to each mechanism? (d) if the conducting material contains two types of charge carriers, a and b, and each carrier type has its own resistivity, pa and pb, what is the total resistivity p?