If a company selects either of Project 1 or Project 2 (or both), then either Project 3 or Project 4 (or both) must also be selected. Which of the following constraints enforce this condition?

a. X1 + X2 ≤ 2(X3 + X4)

b. X1 + X2 ≤ X3 + X4

c. X1 − X3 = X2 − X4

d. X1 + X2 + X3 + X4 ≤ 2