Suppose Xi $($i $=1,2)$ is $1$ if project i is selected and $0$ if project i is not selected. To ensure that project $1$ cannot be selected unless project $2$ is also selected, one would include which of the following constraints?

Group of answer choices

X$1$ X$2<=0$

X$1+$ X$2=1$

X$1$ X$2=1$

X$1+$ X$2<=1$