Working the problem we quickly see that we have r³/3r times 1/k = 1. R divides out of the left side of the equation and we are left with the product of r²/3 and 1/k yielding 1.  Look at the problem carefully.  1/k must be the reciprocal of r²/3 for this product to be 1. If 1/k is equal to 3/r² then, since k is a denominator, it must be the reciprocal or r²/3 . A is the correct answer.
 
 

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