Which of the following statements are true about the Black-Scholes model of option valuation?

Check all that apply:

- The stock price of the underlying asset is directly observable.

- It assumes that the risk-free interest rate is constant.

- It assumes that the volatility of the underlying asset is constant.

- The strike price, time to expiration and standard deviation of the underlying asset's return are directly observable.

- It assumes that the dividend yield of the underlying asset is constant.