18. Consider an ideal lossless coaxial cable transmission line. The dielectric material between the two conductors of the coaxial cable is with a dielectric constant ε = 2.25 ε₀. The cable is open-circuited at one end and is excited with a sinusoidal signal at 1 GHz from the other end. We want to determine at which position inside the cable, the root-mean-square (rms) of the net voltage will have the minimum value. If the distance between the nearest minimum rms voltage point and the open-circuited end is L, please determine which is the closest value for L among the following choices:
(A) 4 cm
(B) 5 cm
(C) 6 cm
(D) 7 cm