Suppose that two unbiased coins are tossed repeatedly and after each toss the accumulated number of heads and tails that have appeared on each coin is recorded. Let Xn be the difference in the accumulated number of heads on coin A and coin B after the nth toss, i.e., Xn = (Total number of heads on coin A) - (Total number of heads on coin B). Thus, the state space S = {0, ±1, ±2, · · · }. Show that the zero state, where the total number of heads equal on each coin, is null recurrent.