Which expression finds the number of different countries represented in the CUSTOMERS table?

The CUSTOMERS table has these columns: The CUSTOMER_ID column is the primary
key for the table. You need to determine how dispersed your customer base is. Which
expression finds the number of different countries represented in the CUSTOMERS table?

A.
COUNT(UPPER(country_address))

B.
COUNT(DIFF(UPPER(country_address)))

C.
COUNT(UNIQUE(UPPER(country_address)))

D.
COUNT DISTINCT UPPER(country_address)

E.
COUNT(DISTINCT (UPPER(country_address)))