Which of the following statements is true about the empirical evidence for the Black-Scholes model?

1. Before the stock market crash of October 1987, the relationship between the implied volatility and the strike price was downward-sloping.

2. After the stock market crash of October 1987, the relationship between the implied volatility and the strike price was flat.

3. Option prices generated by the model tend to be higher than the market prices.

4. Options with higher strike prices tend to have lower implied volatility.