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A letter lock consists of 4 rings, each ...

A letter lock consists of 4 rings, each ring contains 9 non-zero digits. This lock can be opened by setting a 4 digit code with the proper combination of each of the 4 rings. Maximum how many codes can be formed to open the lock?

A

`4^9`

B

`^9P_4`

C

`9^4`

D

none of these

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The correct Answer is:
C
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