In trapezium ABCD,AB|| CD and AB = 2CD.Its diagonals intersect at O. If the area of $$ \triangle AOB =  84  cm^2$$, then the area of $$ \triangle $$COD is equalto
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A

$$42 cm^2$$

B

$$21 cm^2$$

C

$$72 cm^2$$

D

$$26 cm^2$$