Explain why (a) A single central bank and currency for the countries of the European Union mean

Question:

Explain why
(a) A single central bank and currency for the countries of the European Union mean that its members can no longer have an independent monetary policy.
(b) There is no such thing as an among member nations. Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...