The previous two people are estimated to be practical, and their theories are not solid. This problem can be arbitrage, and finally get a net income of 0.79 yuan.

20- 18×e^(- 10%× 1)-3=0.7 1； 0

Buy call options and sell stocks.

It costs 3 yuan to buy a call option and 20 yuan to sell a stock, so you need to borrow 20-3= 17 from others.

Lend 17, and receive17× e (10 %×1) =18.79 yuan one year later.

At this time, when the option expires, the strike price is 18, and you only need 18 yuan to buy the stock again, which is more than 18.79- 18=0.79 yuan.

If you are still unclear, you can read the classic book Introduction to Futures and Options Market written by Hull, which is explained on page P 194 in the fifth edition.