What is a varistor?

ANS:A - a voltage-dependent resistor

The forward and reverse biased states of a diode can be compared to a variable resistor. A forward biased diode will drop only .3 or .7 V.

That corresponds to an internal resistance in the anode that drops from several Kilo-ohms at the point where conduction begins to several ohms where a diode is conducting heavily.

In the reverse bias state, resistance will be in the Mega-ohms until breakdown voltage is reached.