18. [0.5 Points] Below is a plot of f (x) = sin (kx) : 0.5. X -0,5 -0.5 -0.25 0.25 0.5 How does k relate to the wavelength of this function? (Choose all that apply) A) k = 1 B) k = 1/ x C) k = x/2 D) k = 21 . A E) k = 2x/2 F) k = 1/2x