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On the occasion of Deepawali festival, e...

On the occasion of Deepawali festival, each student of a class sends greeting cards to the others. If there are 20 students in the class, then the total number of greeting cards exchanged by the students is

A

`A."^20C_2`

B

`B`.2.`"^20C_2`

C

`C.0`

D

`D`.none of these

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