The keyless entry system in some cars uses a 4-digit keypad. There are 10 possible digits, and the digits can be repeated. How many different combinations are there?

Each of the 4 keypad positions can be filled by any of the 10 digits. Since digits may be repeated, there are 10 × 10 × 10 × 10 = 10,000 possible combinations. The correct choice is C, 10,000.

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