For a certain first$-$order reaction, $AB+C,$ the initial concentration of A was $0\mathrm{.}40M.$ After $20$ seconds, the concentration is $0\mathrm{.}28M.$

Calculate the rate constant.

$1\mathrm{.}8{10}^{-2}{s}^{-1}$

$-1\mathrm{.}8{10}^{-2}{s}^{-1}$

$5\mathrm{.}4{10}^{-2}{s}^{-1}$

$5\mathrm{.}4{10}^{-3}{s}^{-1}$

$-1\mathrm{.}8{10}^{-3}{s}^{-1}$