Consider an American put option on a stock. The stock price is $65.75, the time to maturity is 9 months, the risk-free rate is 5% per annum, the exercise price is $60. If the value of the put option is $1.50, what is the implied volatility of the underlying stock? 1) 17.86% 2) 16.78% 3) 18.76% 4) 18.67%