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If the sum of first n terms of a G. P. i...

If the sum of first n terms of a G. P. is ` alpha` and the sum of its first 2n terms is 5`alpha` :, then the sum of its first 3n terms is

A

`10alpha`

B

`15 alpha`

C

`21 alpha`

D

`25 alpha`

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The correct Answer is:
C
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