In a BIP problem, which of the following constraints will ensure that at most 3 of the 4 possible projects are accepted?

Multiple Choice

x1 + x2 + x3 + x4 ≥ 1

x1 + x2 + x3 + x4 ≤ 3

x1 + x2 + x3 + x4 = 3

x1 + x2 + x3 = x4

None of the answer choices is correct.