In a multiple regression model with 4 explanatory variables and no intercept which one is an unbiased estimate of sigma squared? Time runnin: Attempt due: 1 55 minutes, a. 2 = Li-1 /(n - 4 - 1) b. 2 = i=1 }/(n 2) c. 2 = ?-1/(n 4) d. 2 = $1=1 a /(n) i=1 Ob Od