If tan θ₁ tan θ₂ = k, then (cos(θ₁ – θ₂) / cos(θ₁ + θ₂)) = (a) (1 + k)/(1 – k)  (b) (1 – k)/(1 + k)  (c) (k + 1)/(k – 1)  (d) (k – 1)/(k + 1)