Answer the following questions : (a) Why is direct current (DC) not used to measure the resistance of an ionic solution ? (b) Why are the products of electrolysis different for the electrolysis of aqueous solution of $ ext{AgNO}_3$ with silver electrodes and electrolysis of aqueous solution of $ ext{AgNO}_3$ with platinum electrodes ? (c) Why are magnesium blocks fixed to the iron pipelines carrying water ?