In a BIP problem, which of the following constraints will enforce a contingent relationship between project 1 and 2 such that project 1 can be accepted only if project 2 is also accepted (but project 2 could be accepted without project 1)?

Multiple Choice

x1 + x2 ≤ 1

x1 + x2 = 1

x1 ≤ x2

x2 ≤ x1

None of the answer choices is correct.