Content Area Let x1 , x2 , and x3 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? x1 + x2 + x3 2 x1 + x2 + x3 = 2 x1 + x2 + x3 2 x1 x2 = 0