The standing wave ratio on a $50 \; \Omega$ lossless transmission line terminated in an unknown load impedance is found to be $2.0$. The distance between successive voltage minima is $30 \mathrm{~cm}$ and the first minimum is located at $10 \mathrm{~cm}$ from the load. $Z_L$ can be replaced by an equivalent length $l_m$ and terminating resistance $R_m$ of the same line. The value of $R_m$ and $l_m$, respectively, are
- $R_m=100 \; \Omega, l_m=20 \mathrm{~cm}$
- $R_m=25 \; \Omega, l_m=20 \mathrm{~cm}$
- $R_m=100 \; \Omega, l_m=5 \mathrm{~cm}$
- $R_m=25 \; \Omega, l_m=5 \mathrm{~cm}$