Question (4p): Which of the following expressions represents a spreading wave motion? O Please choose one: OOO a) f(z, t) = 3e-(z-vt) b) f(z,t) = ln(z) e-vt c) f(z, t) = A(coskz - coswt) d) f(z,t) = -2sin (wt + )sinkz a. f(z,t) = -2sin (wt + )sinkz b. f(z,t) = ln(z) et C. f(z,t) = 3e-(z-vt) d. f(z, t) = A(coskz - coswt)