【求助】排列组合牛人进!!!!!!!!!!!!!!

能量毛豆儿

94.   10331-!-item-!-187;#058&007190
A company plans to assign identification numbers to itsemployees.  Each number is to consist offour different digits from 0 to 9, inclusive, except that the first digitcannot be 0.  How many differentidentification numbers are possible?
(A) 3,024
(B) 4,536
(C) 5,040
(D) 9,000
(E) 10,000
Each number is to consist of four different digits  為四位數

the first digit cannot be 0 故有1到9 九個數字
the SECONDdigit 可以為0但是因為the first digit用到一個數字 所以剩下九個數字
the THIRDdigit 剩下八個數字《------为什么不是10个，没说数字不能重复啊
the fourth digit 剩下七個數字
9×9×8×7=4536

tomatoshirt

four different digits　《－－－－

ada7603

楼上正解