A company wants to select only 1 project from a set of 4 possible projects. Which of the following constraints ensures that only 1 project must be selected?
x_(1) x_(2) x_(3) x_(4)<=1
x_(1) x_(2) x_(3) x_(4)=1
x_(1) x_(2) x_(3) x_(4)>=1
x_(1) x_(2) x_(3) x_(4)>=1