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# Consider the reaction ,$2A +B \to products$ when concentration of B alone was doubled , the half -life did not change . When the concentration of A alone was doubled, the rate increased by two times. The unit of rate constant for this reaction is :

$(a)\;L\; mol^{-1}S^{-1} \\ (b)\;\text{No unit} \\(c)\;mol\;L^{-1}\;S^{-1} \\(d)\;S^{-1}$

$2A+B \to product$
when concentration of B is doubled, the half- life did not change , hence reaction is of first order w.r.t B.
when concentration of A is doubled, reaction rate is doubled.
Hence reaction is first order with respect to A.
Hence overall order of reaction is $1+1=2$
So, unit of rate constant is $mol^{-1}\;litres$
Hence a is the correct answer.