For a diode, the plate current is 20 mA when the plate voltage is...

Since the plate current is constant at $20 \text{ mA}$ for plate voltages of $50 \text{ V}$ and $60 \text{ V}$, the diode has reached its saturation state.

In the saturation region, the plate current becomes independent of the plate voltage and depends only on the temperature of the cathode.

Therefore, when the plate voltage is increased to $70 \text{ V}$, the plate current will remain at the saturation value of $20 \text{ mA}$.