The Main Difference Between an American and a European Option

The main difference between an American and a European option is that

A) European options are priced in Euros and American options are priced in dollars.
B) American options can be exercised at any time until expiration while European options can only be exercised on expiration date
C) European options can be exercised at any time until expiration while American options can only be exercised on expiration date
D) American options are standardized contracts while European options are not.