9. Consider all 9-digit integers made by using all the digits 1,2,…,9. Write each such number on a separate sheet and put all the resulting sheets in a box. What is the minimum number of sheets that you must extract from the box if you want to be certain that there are at least two numbers with the same digit in the first place among the chosen numbers?
A) 9! B) 8! C) 72 D) 10 E) 9
I don't get this at all^^... please help