4. (2 marks) Consider the wave equation with free ends J²u J²u 0 < x <π, at² მე2 ди ди (0, t) (π, t) = 0. მე მე Which of the following is a particular solution of this boundary value problem? (a) u(x,t) = et sin(2x) == (b) u(x,t) = et cos(2x) (c) u(x,t) = cos(2t) sin(2x) (d) u(x,t) =sin(2t) cos(2x)