18. The figure shows a thin film (in the middle) with different materials on either side: n₁ = 1.40, n₂ = 1.50, and n₃ = 1.60. We send in a ray of light that is perpendicular to the two parallel surfaces. In air, that light has a wavelength of 800 nm. For some values of thickness L, the reflections to the left undergo fully destructive interference. What is the second smallest value of L that gives that result? Answer in nanometers. (A) 1200, (B) 180, (C) 400, (D) 75, (E) 533.