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The number of all three element subsets ...

The number of all three element subsets of set `{a_(1),a_(2),a_(3),.......,a_(n)}` which contain `a_(3)` is

A

`""^(n)C_(3)`

B

`""^(n+1)C_(3)`

C

`""^(n-1)C_(2)`

D

`2^(n)`

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The correct Answer is:
C
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