in a multiple regression problem involving two independent variables, if b1 is computed to be 2.0, it means that group of answer choices the estimated mean of y is 2 when x1 equals zero. the estimated mean of y increases by 2 units for each increase of 1 unit of x1, holding x2 constant. the relationship between x1 and y is significant. the estimated mean of y increases by 2 units for each increase of 1 unit of x1, without regard to x2.