What Is the Primary Difference Between an American Call Option

What is the primary difference between an American call option and a European call option?

A) The American call has a fixed strike price while the European strike price varies over time.
B) An American call is a right to buy while a European call is an obligation to buy.
C) An American call has an expiration date while the European call does not.
D) An American call is written on 100 shares of the underlying security while the European call covers 1,000 shares.
E) An American call an be exercised at any time up to the expiration date while the European call can only be exercised on the expiration date.