Let $x_1,x_2,$ and $x_3$ be $0-1$ variables whose values indicate whether the projects are not done $(0)$ or are done $(1).$ Which answer below indicates that at least two of the projects must be done? $x_1+x_2+x_3=2x_1-x_2=0x_1+x_2+x_{3}2x_1+x_2+x_{3}2$