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If x, y and r are positive integers , then : `""^xC_r+""^rC_(r-1)""^yC_1+^xC_(r-2)""^yC_2+..........+""^yC_r` =

A

`(x 1. y !)/(r !) `

B

`((x+y) !)/(r !) `

C

`(x+y) C_(r) `

D

`(x y) C_(r) `

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The correct Answer is:
C
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