In a multiple regression model with 4 explanatory variables and no intercept which one is an unbiased estimate of sigma squared? - 1 =1 a. 2 = 1=1 } /(n 4 1) i ( b. 2 = i=1 }/(n 2) c. 2 = 21=1 }/(n 4) d. 2 = 1=1 u/(n) = n- Zi1 Ob Od