In a binary integer programming model, the constraint (x1 + x2 + x3 + x4 = 3) means that: the first three options must b

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In a binary integer programming model, the constraint (x1
+ x2 + x3 + x4 = 3) means that:
the first three options must be selected but not the fourth one
at least three options need to be selected
exactly 1 out of 4 will be selected
exactly three options should be selected
Which of the following best describes the constraint: both A
and B?
B - A = 0
B - A ≤ 0
B + A = 1
B + A ≤ 1