$2)$ Given that String$[]$ str has been initialized, to get a copy of str$[0]$ with all characters converted to upper case, use the following statement:

A$)$ str$.$uppercase$()$;

B$)$ str$[0].$upperCase$()$;

C$)$ str$.$toUpperCase$()$;

D$)$ str$[0].$toUpperCase$()$;