Chloe must pick a 6-digit personal identification number for her bank card. each digit must be between 0 and 9. which expression represents the number of different combinations of 6-digit numbers she can create? | a: 6+10 b: 6x10 c: 10x10x10x10x10x10 d: 6x6x6x6x6x6x6x6x6x6