翻译：$$1000$$至$$9999$$中有多少个位数字互不相同的四位数？  组数问题：千位数字有$$1\sim 9$$共$$9$$种选法，百、十、个位分别有$$9$$、$$8$$、$$7$$种选法，一共有$$9\times 9\times 8\times 7=4536$$（个）数字互不相同的四位数． 
2015年全国$$AMC$$小学高年级六年级竞赛$$8$$第$$10$$题 How many integers between $$1000$$ and $$9999$$ have four distinct digits? 
Options:
[1] $$3024$$ 
[2] $$4536$$ 
[3] $$5040$$ 
[4] $$6480$$ 
[5] $$6561$$ 