Q 1384601557.     `sum_{r=0} ^{n}r *text()^nC_r= ?`

A

`2^{n-1}`

B

`2^n`

C

`n.2^n`

D

`n.2^{n-1}`

HINT

`sum_{r=0} ^{n}r *text()^nC_r=sum_{r=1} ^{n}r*{n}/{r}*text()^{n-1}C_{r-1}`

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