What is the average (mean) of all $$5-$$digit numbers that can be formed by using each of the digits $$1$$, $$3$$, $$5$$, $$7$$, and $$8$$ exactly once? ($$2005$$ AMC $$10B$$ Problem, Question \#$$20$$) 
选项为:A:$$48000$$  B:$$49999.5$$  C:$$53332.8$$  D:$$55555$$  E:$$56432.8$$  请回答其中一个选项.