Let ri be the return for security i, Ri the excess return, and rM and RM the same for the market index. Which regression equation is often used in practice to estimate i? ri(t) = i + i rM(t) + ei(t) ri(t) = i rM(t) + ei(t)M Ri(t) = i + i RM(t) + ei(t) Ri(t) = i RM(t) + ei(t)