There is a three-digit number \textquotesingle$$502$$\textquotesingle. John writes a digit before this number to make it a new four-digit number. If he writes a~\uline{~~~~~~~~~~}~, the new four-digit number would be as small as possible. 
选项为:A:$$9$$  B:$$5$$  C:$$1$$  D:$$0$$  请回答其中一个选项.