Geraldine is picking a four-digit password by using the digits 0 through 9. She can use each digit only once. How many different passwords are possible?

For the first position, Geraldine can choose any of the 10 digits. Since each digit can be used only once, there are then 9 choices for the second position, 8 for the third, and 7 for the fourth. Thus, 10 × 9 × 8 × 7 = 5,040 different passwords, which is choice C.

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