In the Black-Scholes Setting,the Prices of American Options

In the Black-Scholes setting,the prices of American options

A) Cannot be determined because the model assumes away the human impatience that leads to early exercise of options.
B) Cannot be determined because the model does not describe the intermediate stock prices that are required to decide on whether to exercise an option early.
C) Cannot be determined because the model assumes a continuous-time process,whereas to determine early-exercise you need a discrete-time model.
D) Cannot be determined in closed-form.