A transmission line of characteristic impedance $50\; \Omega$ is terminated by a $50\; \Omega$ load. When excited by a sinusoidal voltage source at $10\; \mathrm{GHz}$, the phase difference between two points spaced $2 \mathrm{~mm}$ apart on the line is found to be $\pi / 4$ radians. The phase velocity of the wave along the line is
- $0.8 \times 10^8 \mathrm{~m} / \mathrm{s}$
- $1.2 \times 10^8 \mathrm{~m} / \mathrm{s}$
- $1.6 \times 10^8 \mathrm{~m} / \mathrm{s}$
- $3 \times 10^8 \mathrm{~m} / \mathrm{s}$