Suppose that you enter into a long futures contract to buy December silver for $25.30 per ounce on the New York Commodity Exchange. The size of the contract is 5,000 ounces. The initial margin is $6,000 and the maintenance margin is $4,500. a) What changes in the futures price will lead to a margin call? b) Under what circumstances could $2,500 be withdrawn from the margin account?