The index model has been estimated for stock A with the following results:

$R_A=0\mathrm{.}01+0\mathrm{.}8R_M+e_A.$

${}_M=0\mathrm{.}20$;$(e_A)=0\mathrm{.}10.$

The standard deviation of the return for stock $A$ is

$0\mathrm{.}6400$

$0\mathrm{.}1887$

$0\mathrm{.}1600$

$0\mathrm{.}0356$