Suppose that a rational investor thinks about investing in a portfolio of $3$ risky assets with the following characteristics:

$E(r_A)=0\mathrm{.}06,E(r_B)=0\mathrm{.}08,E(r_C)=0\mathrm{.}15$

${}_1=0\mathrm{.}18,{}_2=0\mathrm{.}25,{}_3=0\mathrm{.}38,{}_{12}=0\mathrm{.}2,{}_{13}=-0\mathrm{.}4,{}_{23}=0\mathrm{.}50$

Construct and draw $($on the same graph$)$ an efficient frontier for the case when short$-$selling is NOT allowed and an efficient frontier for the case when short$-$selling is allowed. Briefly explain your approach and comment.