In the Model $Y _ { i } = \beta _ { 0 } + \beta _ { 1 } X _ { 1 } + \beta _ { 2 } X _ { 2 } + \beta _ { 3 } \left( X _ { 1 } \times X _ { 2 } \right) + u _ { i }$

Multiple Choice

In the model $Y _ { i } = \beta _ { 0 } + \beta _ { 1 } X _ { 1 } + \beta _ { 2 } X _ { 2 } + \beta _ { 3 } \left( X _ { 1 } \times X _ { 2 } \right) + u _ { i }$ the expected effect $\frac { \Delta Y } { \Delta X _ { 1 } }$ is

A) $\beta _ { 1 } + \beta _ { 3 } X _ { 2 }$
B) $\beta _ { 1 } .$
C) $\beta _ { 1 } + \beta _ { 3 }$
D) $\beta _ { 1 } + \beta _ { 3 } X _ { 1 }$