Suppose that a project selection problem has the following constraint: "Projects 1 and 2 must either be both selected or both rejected". If Xi is a binary variable denoting whether or not the ith project is selected, then how should that constraint be stated?

options:

a) X1 - X2 0

b) X1 - X2 0

c) X1 - X2 = 0

d) X1 + X2 = 1

e) X1 + X2 1

f) X1 + X2 = 2