An electron in a metal suffers collision and if the probability of it being scattered over an angle dθ is 2p P(θ) sin θ dθ where P(θ) is an arbitrary function of scattering angle θ, then

• 2pP(θ) sin θ dθ = 0
• 2pP(θ) sin θ dθ = 2
• 2pP(θ) sin θ dθ = -1
• 2pP(θ) sin θ dθ = 1