Suppose that the index model for stocks A and $B$ is estimated from excess returns with the following results:

$R_A=2\mathrm{.}5\%+0\mathrm{.}60R_M+e_A$

$R_B=-1\mathrm{.}5\%+0\mathrm{.}70R_M+e_B$

${}_M=($