A company wants to select 1 project from a set of 4 possible projects. Which of the following constraints ensures that only 1 will be selected?

a. X1 + X2 + X3 + X4 = 1
b. X1 + X2 + X3 + X4 = 1
c. X1 + X2 + X3 + X4= 1
d. X1 + X2 + X3+ X4= 0