In the model y, = 8 0+ 81X 1 + 82X/2+ ..+ Birk + my, the explanatory variables, X/ = (X 1, X/ 2 ...., xx), are sequentially exogenous if. O a E(utxy . Xy-1. ...... ) = E(up) > 0. 1= 12,. . . O b E(upxy . X/-1. ......) * E(up) = 0, 1= 1,2, .... O c. E(upx, . X,-1. ......) = (up) =0, 1= 1,2,... O d E(upxy, Xy-1. ......) = E(up) = 1, 1= 1,2