Easy
If $x, y$ and $r$ are positive integers,then $^x{C_r} + ^x{C_{r-1}} ^y{C_1} + ^x{C_{r-2}} ^y{C_2} + \dots + ^y{C_r} = $
- A$\frac{x! y!}{r!}$
- B$\frac{(x+y)!}{r!}$
- C$^{x+y}{C_r}$
- D$^{xy}{C_r}$
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