If the coefficient of rth term and (r + ...

If the coefficient of rth term and (r + 1)th term in the expansion of `(1 + x)^(3n)` are in the ratio 1 : 2, then r =

A

`(6)/(5) (n + 1)`

B

`(1)/(3) (3n + 1)`

C

`(1)/(4) (n + 2)`

D

`(1)/(3) (3n + 2)`

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