Gregor forms two numbers with digits $$1$$, $$2$$, $$3$$, $$4$$, $$5$$, and $$6$$. Both numbers have three digits, and each digit is used only once. He adds these two numbers. What is the greatest sum Gregor can get? 
选项为:A:$$975$$  B:$$999$$  C:$$1083$$  D:$$1173$$  E:$$1221$$  请回答其中一个选项.