How many of the numbers $$1$$, $$2$$, $$3$$, $$4$$, $$5$$, $$6$$, $$7$$, $$8$$, and $$9$$ can be written as a sum of exactly two of the other numbers? 
选项为:A:$$5$$  B:$$6$$  C:$$7$$  D:$$8$$  请回答其中一个选项.