In a multiple regression problem involving two independent variables, if b1 is computed to be +2.0. It means that a. the estimated mean of Y increases by 2 units for each increase of 1 unit of X1, without regard to X2.
b. the estimated mean of Y increases by 2 units for each increase of 1 unit of X1. holding X2 constant.
c. the estimated mean of Y is 2 when XI equals zero.
d. the relationship between X1 and Y is significant