4. (30 points) For a non-dividend-paying stock with price 21: (1) The risk-free interest rate is 0 (2) A European 3-month put option on the stock with strike price 20 costs 1.00 (3) A European 6-month put option on the stock with strike price 20.30 costs 0.90 (a) (25 points) Construct an arbitrage portfolio. (b) (5 points) If the stock price is 19 after 3 months and 21 after 6 months, calculate the profit of your arbitrage portfolio in part (a)