4. (20 pts) In a binomial tree model, let S(0) = $100, u = 0.12, d= -0.08, r = 0.04. Find today's price of an American put with strike price $102 expiring after two steps. When should the American put be exercised carly? 4. (20 pts) In a binomial tree model, let S(0) = $100, u = 0.12, d= -0.08, r = 0.04. Find today's price of an American put with strike price $102 expiring after two steps. When should the American put be exercised carly