$13.$ Consider the one$-$factor APT. The standard deviation of return on a well$-$diversified portfolio is $\backslash (20\backslash \%\backslash ).$ The standard deviation on the factor portfolio is $\backslash (12\backslash \%\backslash ).$ The beta of the well$-$diversified portfolio is approximately

a$)0\mathrm{.}60$

b$)1$

c$)1\mathrm{.}67$

d$)3\mathrm{.}20$