Consider the general population model in which $y$ depends upon two $x$ variables:

$$y = \beta_{0} + \beta_{1} x_{1} + \beta_{2} x_{2} + u$$

If we instead estimate the simple regression:

$$\tilde{y} = \tilde{\beta_{0}} + \tilde{\beta_{1}} x_{1}$$

...which factors solely determine the direction of bias in the estimated parameter $\tilde{\beta_{1}}$?

Select **ALL** that apply.