Consider the current–voltage characteristics for the devices shown...

Given $V_A < V_B$, terminal A is at a lower potential than terminal B.

For the left circuit:
The diode is connected with its p-side towards A and n-side towards B. Since $V_A < V_B$, the diode is reverse-biased and acts as an open circuit.
Thus, the equivalent resistance of the left device is $\infty$.

For the right circuit:
The upper branch has a diode with its p-side towards A and n-side towards B. Since $V_A < V_B$, it is reverse-biased and offers infinite resistance.
The lower branch has a diode with its n-side towards A and p-side towards B. Since $V_B > V_A$, it is forward-biased and acts as a short circuit (ideal diode).
The equivalent resistance of the right device is determined solely by the lower branch, which contains a $20\ \Omega$ resistor.
Thus, the equivalent resistance of the right device is $20\ \Omega$.

The equivalent resistances are $\infty$ and $20\ \Omega$ respectively.