In a multiple regression problem involving two independent variables, if b1 is computed to be +2.0, it means that:
Group of answer choices:
A. the estimated mean of Y increases by 2 units for each increase of 1 unit of X1, without regard to X2.
B. the relationship between X1 and Y is significant.
C. the estimated mean of Y increases by 2 units for each increase of 1 unit of X1, holding X2 constant.
D. the estimated mean of Y is 2 when X1 equals zero.