With respect to ANSI, ASME and ISO dimensioning and tolerancing standards one can use any letter designator as a datum other than I, O and Q. You can even use double letters like datum "AA" or "ZF" and even combine numbers with letters as typical with datum targets.
e.g. A1, A2, A3, etc..
Not using "C and D" would be a preference as there is not a rule on the order of letters chosen for datums. I prefer using the letters in the order given by the alphabet however I know this is not a requirement.