The good answer is
C. The selected currency is used for the Amount field
D. Currency must be specified for the opportunity
As mentioned in the documention named "Considerations for Enabling Multiple Currencies"
The first consequence is that currency must be specified before every entry on the transaction. The multiple currencies function also affects the Opportunity field directly. The entries in the opportunity fields must be the local currency of the area where the salesforce is being used.
Other consequences of enabling multi-currency include, ISO codes of all currency being displayed before amounts, the current currency Locale becomes the default currency stamp and existing records get stamped with the existing currency code.
Also, this feature cannot be disabled once it's been enabled.